-
MESO: Heavy rain/flooding
From
Weather Alert@454:1/105 to
All on Thu Dec 31 17:56:00 2020
AWUS01 KWNH 311946
FFGMPD
LAZ000-ARZ000-TXZ000-010100-
Mesoscale Precipitation Discussion 0884
NWS Weather Prediction Center College Park MD
246 PM EST Thu Dec 31 2020
Areas affected...Eastern TX...Western LA...ext Southwest AR...
Concerning...Heavy rainfall...Flash flooding possible
Valid 312000Z - 010100Z
SUMMARY...Multiple bands starting to merge along cold front with
occasional rain-rates up to 1.75"+/hr though with increasingly
progressive cell motions pose possible flash flooding
DISCUSSION...GOES-E WV suite continues to show the inner core of
the closed low shifting eastward with the strengthening eastern
side shortwave. This continues to support solid surface
cyclogenesis near the Houston Metro. This also drawn the
stationary front across LA further south while steepening the
isentropic packing along the front. As such, the very strong
nearly orthogonal boundary layer southeasterly flow and
strengthening 85H southerly to around 45-50kts. This leads to
deeper moisture flux convergence and vertical development and
therefore higher depth of rainfall generation. This also narrows
the TROWAL angling back northwest to the motion of the surface
low. The strong DPVA combined with diffluent 3H flow within the
Left-Exit Region of the 125kt jet should maintain the deepening of
the cyclone and isallobaric component for further rainfall
efficiency as the 850mb front also aligns with the surface
boundary around 22-23z. Given MUCAPES pushing 1000 J/kg and total
PWs around 1.75" and strong flux convergence, rainfall efficiency
of 2"/hr are possible with perhaps some occasional sub-hourly
bursts of higher intensity with merging cells just northeast of
the surface cyclone where streams converge greatest.
Greater veering flow aloft and increasing steering flow as the mid
to upper level jet streams northeast, the low level warm
conveyor/TROWAL should allow slightly elevated remaining
convective elements to stream well north into East-Central TX and
Northwest LA. However, increased speeds will also reduce duration
of the 2"+/hr rates with expectedtotals of 2-3" genearlly along a
line from LFK to JAS to between IER/POE where cells may linger a
bit longer along/just north of the front before upgliding
producing scattered streaks extending northward. This is best
represented by recent HRRR and HRRRv4 solutions. After 00z, the
mid-level forcing wave may begin to overtake the lower-level
surface boundary and start to reduce best deeper vertical ascent
and reduce overall coverage as cells expand into central LA, and
across areas of drier ground conditions, making flash flooding a
bit less possible, though lower FFG values near SHV and into AR
will need to monitored closely.
Gallina
ATTN...WFO...FWD...HGX...LCH...SHV...
ATTN...RFC...LMRFC...WGRFC...NWC...
LAT...LON 33429362 32869264 31129287 29749312 29479443
29969478 30569513 31219526 31779547 32449556
33039542 33389472
#
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From
Weather Alert@454:1/105 to
All on Thu Dec 31 10:52:00 2020
AWUS01 KWNH 311440
FFGMPD
LAZ000-TXZ000-312015-
Mesoscale Precipitation Discussion 0883
NWS Weather Prediction Center College Park MD
940 AM EST Thu Dec 31 2020
Areas affected...Southeast TX...
Concerning...Heavy rainfall...Flash flooding possible
Valid 311445Z - 312015Z
SUMMARY....Intersection of greatest rain-rates/totals are
decoupling from areas of lowest FFG/ground saturation; however,
ingredients for isolated flash flooding rain-rates still remain
possible particularly in urban settings and near the coast through
early after-noon.
DISCUSSION...GOES-E WV depicts a very strong winding up closed low
across Northeast Mexico with very impressive baroclinic leaf
across much of TX. The very sharp distinct mid-level drying at
the nose of stronger height-falls/core of the approaching
shortwave rotating around the base/SE side of the cyclone is
advancing through Northern Nuevo Leon. Very strong DPVA
associated with this shortwave, along with favorable placement of
the 120kt 3H jet core just south of the TX coast provides ample
support for low-level wind increases and surface-low level
cyclogenesis. As such, low level wind response is further
strengthening FGEN forcing along the slowly advancing cold front
that extends from the surface wave near ARM due south, while very
strong warm advection is pumping western Gulf moisture/instability
across coastal Southeast TX (south of the stationary front from
said surface low to s of LFK to AQV to ESF to HEZ. Solid MLCAPE
of 500-750 along with 1.5" total PWATs will be increasing due to
the surface/low level response and perhaps some weak solar
insolation by midday reaching 1000-1250 J/kg and 1.75" and
20-25kts of SSE flow converging with nearly due west flow at the
cold front. Combined with frictional convergence at the coast,
thunderstorms capable will start around 1-1.25"/hr rates possibly
becoming 1.5"-1.7/hr through the early afternoon (18-20z).
While the rainfall environment is deteriorating through the
afternoon, the hydrologic situation may be a bit better.
Currently, the cold front/axis of heavy rainfall is along the
eastern edge of the heavy rainfall footprint from over night where
greater than 5" was observed across Jackson to Waller counties
where FFG is compromised to less than .5" through all time-steps
(1/3/6hrs). This axis continues northeast through Trinity and
Polk counties though lower totals were observed with blow-off
showers from the deeper convection. Still, this is aligned with
strong orthogonal intersection with the stationary front to keep
some isolated convective development lingering and compounding
longer duration totals. With the approach of the shortwave,
there remains a hour or so of slow eastward motion before deeper
upper-level steering flow and height-falls support eastward
propagation vectors of the cold front itself. Cells may continue
to train northward along the cold front but overall motions should
reduce totals at a given location or over saturated areas.
However, this also moves the axis through the urban corridor of
the Houston Metro where naturally greater run-off will occur with
hydrophobic ground conditions...so with 2-3" totals to be
expected, flash flooding remains possible here before further
moving west with the strengthening rates by early afternoon.
Currently, the NMMB has been trending the best both in placement
and magnitude likely due to the greater synoptic forcing over the
thermodynamic forcing, though all Hi-Res CAMs are trending a tad
faster on the cold front reducing totals slightly.
A secondary elevated band of convection currently observed at the
leading edge of the 85-7H front/wind shift associated with the
shortwave aloft, lingering mid-level moisture will continue to
support some moderate showers and perhaps an additional .5" totals
which given that lower FFG values may once again yield some
further flooding concerns. There will be a pivot to the TROWAL
across SE TX that may allow for greater duration if at a lower
rain-rate through upglide process rather than vertical/convective
processes.
Gallina
ATTN...WFO...HGX...LCH...
ATTN...RFC...WGRFC...NWC...
LAT...LON 30949474 30609387 29619388 28989489 28429611
29089631 29539633 30179620 30779549
#
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From
Weather Alert@454:1/105 to
All on Sat Jan 2 11:16:00 2021
AWUS01 KWNH 021453
FFGMPD
FLZ000-GAZ000-022000-
Mesoscale Precipitation Discussion 0002
NWS Weather Prediction Center College Park MD
953 AM EST Sat Jan 02 2021
Areas affected...FL Panhandle... Southern GA
Concerning...Heavy rainfall...Flash flooding likely
Valid 021445Z - 022000Z
SUMMARY... Training band of heavy rain and embedded thunderstorms
to support a growing threat of flash flooding this morning and
into the afternoon hours.
DISCUSSION... GOES 16 IR imagery shows cold cloud tops associated
with deep convection over the Gulf of Mexico. This area of
convection is focused along a quasi-stationary frontal boundary
positioned from South Carolina and southern Georgia extending
southwest through the the Florida Panhandle and into the Gulf of
Mexico. The front is located between a deep upper trough over the
south-central U.S. and a subtropical high over the northern
Bahamas. 12Z sounding out of TLH sampled an environment favorable
for heavy rainfall with a deep warm cloud layer that extended more
than 10 kft up into the column. In addition, 850-300mb mean winds
are out of the SW, oriented parallel to the frontal boundary.
Ample atmospheric moisture is available with PWs of 1.78" and
skinny MUCAPE was also present. This environment is also expected
to translate farther north and east into south-central Georgia
this morning.
As the day progresses, the slow moving nature of the frontal
boundary will remain a trigger for additional heavy rain and
thunderstorms. Available MUCAPE may vary at times but generally
should hover close to 500 J/kg into the early afternoon hours.
Area averaged HRRR soundings early this afternoon suggest PWATs
hover around the 1.8-1.9" range with some guidance indicating PWs
up to 2" possible later in the day. Over the last several hours
some locations have already reported over 2" of rainfall with
hourly rates over 1" within heavier cells. The training of deep
moist convection over these areas are likely to produce additional
rainfall totals between 2-4" inches over the next five hours with
locally higher amounts possible where the most intense
thunderstorms take shape. There are already reports of flash
flooding and with overly saturated antecedent conditions having
already occurred, additional areas flash flooding are likely to
transpire throughout the day.
Mullinax
ATTN...WFO...CHS...FFC...JAX...TAE...
ATTN...RFC...SERFC...NWC...
LAT...LON 32358213 32128172 31768162 31158234 30288360
29708456 29438524 30038557 31308416 32028295
#
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From
Mike Powell@454:1/105 to
All on Thu Aug 3 07:57:00 2023
AWUS01 KWNH 031025
FFGMPD
TNZ000-KYZ000-INZ000-ILZ000-MOZ000-031500-
Mesoscale Precipitation Discussion 0846
NWS Weather Prediction Center College Park MD
624 AM EDT Thu Aug 03 2023
Areas affected...central and southeastern MO into central KY/TN
Concerning...Heavy rainfall...Flash flooding likely
Valid 031023Z - 031500Z
Summary...Areas of heavy rain and flash flooding are likely to
continue from portions of central and southeastern MO into western
KY through 15Z. Farther east, possible flash flooding will exist
for portions of central KY/TN. Additional totals of 2-4 inches are
expected.
Discussion...10Z regional radar imagery continued to show an axis
of training heavy rain over central and southeastern MO, although
the axis is no longer contiguous. Rainfall rates have ranged from
1-3 in/hr across this region of MO since 06Z and portions of the
region have seen 3 to 8 inches of rain over the past 24 hours
(MRMS estimates). Farther east, a well-defined MCV was located in
southern IN, tracking toward the southeast, associated with a
small region of 1-2 in/hr rainfall rates ~30 miles south of
Louisville, KY along I-65. VAD wind plots at 850 mb from KSGF and
KPAH showed 35-40 kt from the west and tied with some of the
coldest cloud tops on IR imagery. However, overrunning low level
flow and areas of confluence were still producing thunderstorms
upstream across central MO where some of the heaviest rain has
fallen over the past 6-12 hours. Precipitable water values were
estimated to be between 2 and 2.4 inches and instability ranged
from a few hundred J/kg in central KY to 1000 to 2000+ J/kg near
the confluence of the OH and MO rivers.
The focus for the heaviest rain over the next 3-5 hours is
expected to set up from southeastern MO into far southern IL and
western KY, in line with the strongest isentropic lift atop a
surface front draped across the central MO/AR border. Areas of NW
to SE training with 1-2 in/hr rates (perhaps locally higher) are
expected to continue areas of flash flooding. Farther upstream,
ascent is expected to diminish over the next few hours, however, a
localized flash flood threat will remain across central MO where
ground conditions are hyper sensitive due to recent heavy rainfall.
Locations farther east from KY into TN, ahead of the MCV, will see
a localized flash flood threat where lift ahead of the MCV will
continue areas of heavy rain with brief training just downstream
of the MCV center. The greatest limiting factor for excessive
rainfall across central KY/TN will be weak instability. However,
peak rainfall rates of 1-2 in/hr may still be enough to support
localized rainfall totals of 2-3 inches through 15Z.
Otto
ATTN...WFO...EAX...JKL...LMK...LSX...MEG...MRX...OHX...PAH...
SGF...
ATTN...RFC...LMRFC...MBRFC...NCRFC...OHRFC...NWC...
LAT...LON 38809182 38199048 37758952 37478873 37348821
37348770 37498725 38138612 38128551 37818483
37348425 36908404 36388417 36108477 35828556
35528656 35448764 35608834 36088934 36949067
37859232 38349296 38789291
= = =
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From
Mike Powell@454:1/105 to
All on Sun Aug 6 08:11:00 2023
AWUS01 KWNH 061300
FFGMPD
MOZ000-MNZ000-IAZ000-SDZ000-061900-
Mesoscale Precipitation Discussion 0876
NWS Weather Prediction Center College Park MD
859 AM EDT Sun Aug 06 2023
Areas affected...Iowa and southern Minnesota
Concerning...Heavy rainfall...Flash flooding possible
Valid 061300Z - 061900Z
Summary...Showers and thunderstorms will increase in coverage this
morning and then drift slowly to the northwest across Iowa into
Minnesota. Rainfall rates of 2"/hr or greater at times are likely,
which through slow motions could produce 2-4" of rain. Flash
flooding is possible.
Discussion...The regional radar mosaic this morning shows
expanding coverage of showers and isolated thunderstorms across
central Iowa. This convection is blossoming downstream of a
stacked low pressure moving eastward out of SD, with downstream
enhanced ascent driven by height falls, impressive PVA, and
modestly coupled upper jet streaks. This ascent will continue to
impinge into favorable thermodynamics characterized by PWs of 1.5
to 1.7 inches measured by morning UA soundings, around the 90th
percentile for the date, collocated with MUCAPE above 1000 J/kg
analyzed by the SPC RAP. Within this environment, rainfall rates
have been estimated via KDMX and KFSD to exceed 2"/hr, and while
this may be a bit overdone, it is reflective of the possible
intensity developing through this aftn.
The stacked low will shift slowly eastward through the day,
maintaining robust deep layer ascent across the area. At the same
time, S/SE flow downstream of the wave should drive PWs to above
1.75" this aftn, coincident with MUCAPE remaining 1000-1500 J/kg.
Despite significant cloud cover which may temper surface-based
instability except in clearings, impressive theta-e advection
lifting cyclonically into a TROWAL should supply ample elevated
instability to persist heavy rain rates. This is likely
manifesting as the high MUCAPE in model output, and when combined
with a well-collocated overlap of the highest PW pool and warm
cloud depths surging above 11,000 ft, will drive extremely
efficient warm rain processes. This will result in rainfall rates
which the HREF indicates could exceed 2"/hr, with the HRRR
suggesting brief 3+"/hr rates noted via the 15-min rainfall
accumulation product. Storms that develop will generally move
slowly NW on 10-15 kt 0-6km mean winds, but increasingly
anti-parallel and collapsing Corfidi vectors suggest considerable
backbuilding potential to result in training and at times storm
motions of just 0-5 kts. Where the most persistent training can
occur, rainfall of 2-4" is possible as shown by the HREF and
experimental RRFS TL ensemble probabilities.
Although 7-day rainfall across northern IA and southern MN has
been well below normal according to AHPS, heavy rain did impact
parts of this area in the past 24-hours. This has resulted in
nearly saturated top-soils according to the HRRR, and locally
compromised FFG as low as 0.75-1.5"/3hrs. While the slow motions
of any intense rainfall rates could result in instances of flash
flooding through the aftn, the greatest risk will be where
training can occur atop these most vulnerable soils.
Weiss
ATTN...WFO...ARX...DMX...DVN...EAX...FSD...MPX...OAX...
ATTN...RFC...MBRFC...NCRFC...NWC...
LAT...LON 44479441 44219309 43479205 42409142 41139128
40439170 40349241 40859342 41439461 41799562
42379619 42889648 43299648 44069639 44379578
= = =
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From
Mike Powell@454:1/105 to
All on Sat Sep 30 08:11:00 2023
AWUS01 KWNH 300932
FFGMPD
FLZ000-301530-
Mesoscale Precipitation Discussion 1117
NWS Weather Prediction Center College Park MD
530 AM EDT Sat Sep 30 2023
Areas affected...Portions of the FL Atlantic Coast
Concerning...Heavy rainfall...Flash flooding possible
Valid 300930Z - 301530Z
SUMMARY...Slow-moving showers and thunderstorms this morning with
locally extreme rainfall rates may produce some areas of flash
flooding.
DISCUSSION...The latest radar imagery has been showing an uptick
in very heavy shower and thunderstorm activity around the Daytona
Beach, FL area and extending offshore with some linear bands
becoming aligned with a convergent low-level flow regime just
north of a quasi-stationary frontal zone.
A very moist and unstable airmass is pooled across the region with
PWs that are running over 2 standard deviations above normal with
values of 2.25 to 2.5 inches. The 00Z RA0B sounding from KXMR
(Cape Canaveral) was reflective of a very deep and warm tropical
column with WBZ heights well over 15,000 feet and a PW of 2.31
inches. The latest RAP analysis shows MLCAPE values of 1500+ J/kg
along the coast and already indicative of a moderately unstable
airmass.
The combination of convergent low-level easterly flow, westerly
shear aloft, and this very moist and unstable airmass will support
slow-moving and locally anchored convective cells focusing along
and adjacent to the coast that will be capable of extreme rainfall
rates.
Some rainfall rates with the more organized convective cells this
morning will be capable of reaching 2 to 4 inches/hour and this
will also be connected to the fact there that is such a deep and
moist warm cloud layer.
The 06Z HREF guidance suggests a threat for localized storm totals
reaching or exceeding 5 inches where the storms become more
focused, and at least in the short-term, areas near Daytona Beach
will be most at risk for seeing these heavier totals. Going
through the morning hours, some additional expansion of convection
especially to the north along the coast may be possible.
Very high FFG values will certainly mitigate any flash flood
threat to an extent, but with such high rainfall rate potential
this morning, some flash flooding will be possible and especially
if any urban areas can get into the stronger convective cores.
Orrison
ATTN...WFO...JAX...MLB...
ATTN...RFC...SERFC...NWC...
LAT...LON 29878130 29828118 29598108 29228087 28808077
28598080 28678113 29008136 29428154 29758153
= = =
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From
Mike Powell@454:1/105 to
All on Thu Oct 26 08:43:00 2023
AWUS01 KWNH 260941
FFGMPD
TXZ000-261540-
Mesoscale Precipitation Discussion 1161
NWS Weather Prediction Center College Park MD
540 AM EDT Thu Oct 26 2023
Areas affected...Much of Central and South-Central TX
Concerning...Heavy rainfall...Flash flooding possible
Valid 260940Z - 261540Z
Summary...Rainfall totals of 1-2" may continue through morning,
possibly leading to additional localized totals of 3-5"+.
Scattered instances of flash flooding are possible, and some may
be significant.
Discussion...Showers and thunderstorms continue early this morning
in the vicinity of a surface trough draped across North and
Central TX. Rainfall rates have moderated a bit over the past
several hours but have shown some recovery as of late, still as
high as 1-2"/hr locally. While progression of the convection
eastward has begun to limit the threat for additional flooding
across northeast TX, there is less eastward progression farther
south (into the TX Hill Country and South-Central TX) which is
facilitating localized training and repeating of 1-2"/hr rainfall
rates. This is in a mesoscale environment characterized by PWATs
of 1.4-1.9" (between the 90th percentile and max moving average,
per surrounding sounding climatology), appreciable instability (as
measured by 500-1000 J/kg of ML CAPE), and effective bulk shear of
20-40 kts.
Hi-res guidance has come into far less agreement through 16z,
despite much better agreement among 00z HREF members earlier
overnight. Both the HREF suite and more recent hourly runs of the
HRRR have struggled to depict the intensity of the southern end of
the convection, but the most recent HRRR runs (since 06z) have
begun to depict a more concerning scenario where a narrow band of
training convection sets up near the I-35 corridor (including the
Austin and San Antonio metro areas) and have shown some southerly
trend. Six hourly totals are indicated to be on the order of 3-9"
in these latest runs, which is certainly plausible given the
highly efficient warm rain processes and potential for training.
That said, the 00z HREF is far less aggressive, generally
suggesting 20-40% probabilities for localized 3" exceedance (per
the 40-km neighborhood method), and negligible (~5%) odds for
localized 5" exceedance. The above introduces above average
uncertainty. Given the concerning trends in the HRRR (with
observational trends supporting these concerns), continued flash
flooding is considered possible (as well as locally significant
flooding, given the potential magnitude).
Churchill/Roth
ATTN...WFO...EWX...FWD...HGX...SJT...
ATTN...RFC...WGRFC...NWC...
LAT...LON 32729592 31519596 30249674 28889793 28869937
29409997 29800000 30899903 31509830 32229726
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From
Mike Powell@454:1/105 to
All on Sat Dec 2 09:55:00 2023
AWUS01 KWNH 021235
FFGMPD
FLZ000-GAZ000-ALZ000-MSZ000-LAZ000-021833-
Mesoscale Precipitation Discussion 1186
NWS Weather Prediction Center College Park MD
733 AM EST Sat Dec 02 2023
Areas affected...Central Gulf Coast into the Florida Panhandle
Concerning...Heavy rainfall...Flash flooding possible
Valid 021233Z - 021833Z
SUMMARY...Showers and thunderstorms will generally be on the
increase in coverage and intensity going through the midday hours.
A combination of heavy rainfall rates and localized training cell
activity may result in scattered instances of flash flooding.
DISCUSSION...The latest surface analysis shows a west/east
oriented low-level trough axis/mesoscale boundary lingering over
the far northern Gulf of Mexico south of the FL Panhandle and
extending westward into the coastal parishes of south-central to
southeast LA. Strengthening deep layer warm air advection and
moisture transport over this boundary in response to a weak
low-amplitude shortwave perturbation advancing northeast across
the central Gulf Coast has resulted in a general uptick in the
coverage of heavy shower and thunderstorm activity.
MUCAPE values of as much as 1000 to 1500 J/kg are noted across
southeast LA and into the immediate offshore waters of southern MS
and southern AL. The 3-hour MUCAPE differentials across this
region are as much as +250 to +500 J/kg, and suggestive of the
northeast transport of greater instability albeit a bit elevated.
The PW values are gradually increasing with time with the NESDIS
Blended TPW imagery showing a well-defined northward transport of
tropical moisture with 2.0 to 2.25 inch PWs lifting north from the
Gulf of Mexico toward the central Gulf Coast. The aforementioned
low-amplitude shortwave perturbation shows up quite well in the
850/700mb and 700/500mb CIRA-ALPW datasets approaching southwest
to south-central LA, and there is evidence in surface observations
of a low center near the coast.
Already there are broken areas of convection with heavy rainfall
rates of 1 to 2 inches/hour impacting parts of south-central to
southeast LA and coastal areas of southern MS. Even stronger
convection with heavier rates are noted offshore of the FL
Panhandle to the south and southwest of Panama City as a more
organized convective band with very cold cloud tops aligns itself
in close proximity to the aforementioned mesoscale boundary/trough.
Given the increasingly parallel orientation of the convection to
the deeper layer mean flow, there will be concerns for some
training convective bands/cell-activity that will favor excessive
rainfall totals. The 06Z HREF and recent HRRR guidance may be a
bit too far north with their QPF footprint this morning, and the
latest radar/satellite trends suggest areas closer to the Gulf
Coast itself will tend to be at risk for the heaviest amounts.
Additional rainfall amounts of as much as 3 to 5 inches will be
possible going through midday, and this may result in scattered
instances of flash flooding and especially around the more
sensitive urbanized locations.
Orrison
ATTN...WFO...JAN...LCH...LIX...MOB...TAE...
ATTN...RFC...LMRFC...SERFC...NWC...
LAT...LON 31398750 31188539 30868451 30328396 29738424
29528504 29918577 30188672 30088841 29628958
29369064 29309158 29809183 30399105 31138911
= = =
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From
Mike Powell@454:1/105 to
All on Thu Dec 21 19:24:00 2023
AWUS01 KWNH 212131
FFGMPD
CAZ000-220330-
Mesoscale Precipitation Discussion 1213
NWS Weather Prediction Center College Park MD
430 PM EST Thu Dec 21 2023
Areas affected...Portions of the Central and Southern California
Coast
Concerning...Heavy rainfall...Flash flooding likely
Valid 212130Z - 220330Z
SUMMARY...The flash flooding threat continues along portions of
the central and southern California coast. The heaviest rain will
shift west, resulting in diminishing rainfall rates from east to
west through this afternoon.
DISCUSSION...Heavy rainfall continued this morning primarily
through the Santa Barbara area, where considerable flash flooding
is underway, including flooding of homes and businesses in the
Santa Barbara area. The corridor of heaviest rain remains near
Santa Barbara, and is likely to persist over the next several
hours as it only slowly drifts westward. 6 hourly rainfall totals
of 2 to 4 inches have been reported in and around Santa Barbara,
with considerably lighter amounts both east and west, showing how
narrow a corridor of heavy rain this has been so far.
The upper level shortwave forcing supporting the heaviest rainfall
and circulating around the broader upper low is moving offshore to
the west. MUCAPE values have only diminished to between 250 and
500 J/kg. CAMs guidance is in good agreement that on the whole,
the heaviest rainfall within the corridor should gradually
diminish with time. All of these factors are likely to result in
more intermittent periods of heavy rain with rates up to an inch
per hour, but the training and painfully slow broad movement of
the heavy rain corridor will keep the flash flooding threat in the
likely category through the afternoon, particularly around Santa
Barbara.
As the corridor of heaviest rain shifts west late this afternoon
and evening, increasing easterly/offshore/downslope flow should
result in a sharp cutoff on the eastern side of the precipitation
shield. The upper level low driving all this rainfall is moving
southeastward well off the coast of Mexico, which will result in a reorientation of the rain bands from the current SSE-NNW to SE-NW
this evening, and eventually E to W by late tonight. This in turn
will result in diminishing rainfall for areas further north into
Monterey and SLO Counties, though coastal sections of those
counties may continue to see heavy rain well into the late
afternoon. However, this same trend may allow bands of heavy rain
to impact the remainder of the CA coast down to the Mexico border
late tonight.
Wegman
ATTN...WFO...HNX...LOX...MTR...
ATTN...RFC...CNRFC...NWC...
LAT...LON 36282129 36192061 36102037 36092022 35952002
35571954 35151904 34831880 34461876 34021906
34351958 34412052 35082071 35532122 36072166
= = =
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From
Mike Powell@454:1/105 to
All on Sat Feb 10 08:47:00 2024
AWUS01 KWNH 101042
FFGMPD
LAZ000-TXZ000-101641-
Mesoscale Precipitation Discussion 0055
NWS Weather Prediction Center College Park MD
542 AM EST Sat Feb 10 2024
Areas affected...portions of southeast TX & western LA
Concerning...Heavy rainfall...Flash flooding possible
Valid 101041Z - 101641Z
Summary...A low-level convergence zone is expected to gather an
increasing amount of thunderstorm activity through the morning
hours. Hourly rain totals to 2" with local amounts to 4" are
possible.
Discussion...A shortwave moving near El Paso has been enhancing
outflow aloft across much of central and eastern TX and slowly
eroding CIN. Precipitable water values near the TX coast have
risen into the 1.5-1.75" range per GPS data. MU/ML CAPE has been
more or less stable near 250 J/kg on SPC mesoanalyses. A
low-level convergence zone, occasionally showing up in the surface
pattern but more evident at 925 hPa and 850 hPa, has been moving
little near the Middle and Upper TX coasts and across western LA
within the warm sector of a low over TX, which helped focus a band
of showers hours ahead of what was suggested by the 00z HREF.
Around 3 a.m. CST, there was a spike in hourly rain totals up to
1.5", also earlier/more than anticipated by the guidance, from the Galveston/Harris County border into southeast Liberty County.
Various model fields suggest that instability will be stable or
slowly increase initially, before going upward faster later this
morning near the southern end of the low-level convergence zone
either due to daytime heating or the approaching shortwave. A
boundary itself may shift slightly northward. The past few HREF
runs have been slowly trending to a wetter signal across this
region, focusing more on this convergence zone and less on the
front to the north. As the shortwave near westernmost TX
approaches, the expectation is for convective coverage to increase
near the convergence zone, particularly after 13z per the 00z/06z
HREF guidance is to be believed. The 00z ARW appears best placed
with what's expected, based on what's occurred thus far, though
its amounts could be a little light. Hourly rain totals are
expected to peak in the 2" range, with local amounts to 4" being
possible, which would exceed the three hour flash flood guidance.
Any heavy rain-related issues would be isolated to widely
scattered, with urban areas most sensitive.
Roth/Asherman
ATTN...WFO...HGX...LCH...SHV...
ATTN...RFC...LMRFC...WGRFC...NWC...
LAT...LON 31799320 31209252 30029415 28999604 29509646
30789509
= = =
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