April 2021

General Weather Discussions and Analysis
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DoctorMu
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don wrote: Mon Apr 19, 2021 11:46 am Models are showing possibly multiple rounds of heavy rain and severe thunderstorms Friday and Saturday. As the frontal boundary briefly stalls which allows multiple disturbances to ride the front over the area.
Big Friday afternoon and evening event for us. All about timing.
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jasons2k
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Great discussion:
Area Forecast Discussion
National Weather Service Houston/Galveston TX
340 PM CDT Mon Apr 19 2021

.DISCUSSION...

.SHORT TERM [Through Tuesday Night]...

The quiet weather pattern continues today thanks to surface high
pressure sitting over Southeast Texas. As the high slides off to
the east, winds will transition from easterly to southeasterly.
Temperatures today will be slightly below normal with highs in the
low-to-mid 70s. For tonight, temperatures will be a bit warmer
than the previous night since CAA has come to an end. Overcast
clouds moving in overnight will initially keep temperatures on the
mild side, but these will depart after midnight allowing
temperatures to drop into the low 50s. It`s worth mentioning that
some hi-res models are indicating isolated rain showers developing
after sunset due to upper-level divergence from a departing jet
streak, but with plentiful dry air in the lowest 3km above the
surface, any precipitation that falls will evaporate before it
reaches the ground. Thus, PoPs will remain less than 10%
throughout the night.

Southwesterly winds kick in and skies clear out on Tuesday morning
as surface high pressure continues its eastward trek. Resultingly,
afternoon temperatures will be slightly warmer with highs
approaching the 80 degree mark at most locations. The southwesterly
winds will gradually decrease our PW values down into the 0.6 inch
to 0.8 inch range, so our moisture profile looks relatively slim
ahead of our next cold front that will pass through the region on
Tuesday afternoon/evening. Since there will be dry air out ahead
of the front, models are in agreement that this will be a dry
FROPA. With sufficient CAA persisting through Tuesday night,
temperatures will be at least 10-15 degrees below normal with lows
ranging from the low 40s to the low 50s across Southeast Texas.

.LONG TERM [Wednesday Through Monday]...

Wednesday morning puts us in a decidedly post-frontal environment,
as a surface high sits over the Red River Valley. The cold
advection does not look particularly strong, but we also weren`t
starting from the warmest, most humid place either, so Wednesday
still looks to be unseasonably cool and dry. Of course, we are
now arriving in the time of year where your reaction to that
sentence depends wholly on how you feel about heat and humidity.
For this Wisconsin-raised forecaster, a sunny day with highs in
the upper 60s to 70s with dewpoints ranging from the upper 30s to
lower 50s sounds downright wonderful! But your mileage may vary.

Spring being a season of transition, this setup won`t stay static,
particularly since the flow aloft is relatively zonal. Wednesday
night will see winds veer to become more easterly, picking up a
connection with more humid Gulf air. Meanwhile, a surface low
should be starting to spin up over New Mexico underneath what
looks like it will be a pretty robust shortwave trough dropping
off the Rockies. This will keep the pressure gradient fairly
tight, for the back half of the week, aiding the return of deep
moisture over the area.

Depending on how quick moisture return is, we could see some low
chances for showers as early as late Thursday afternoon down by
Matagorda Bay closer to the deep moisture axis. However, by Friday
morning, the potential for showers will spread across our area.
Depending on how significant capping is, we could see some
afternoon convection on Friday, particularly if we see some pre-
frontal surface troughing as a focus mechanism, as hinted at in
at least some of the guidance. The other focus will come later
Friday night into Saturday as the front sweeps through. Though the
timing is not best, there looks to be enough instability that we
could sustain some existing, organized severe weather for some
time right on the front. SPC does have the western half or so of
our area in what is the equivalent of a slight risk area for
Friday night, and that seems a reasonable forecast. For what is a
Day 5 forecast, there`s only so much that can be divined from what
will eventually be driven by mesoscale details.

Speaking of mesoscale details, the GFS has a party with them...or
at least the best approximations of them it can resolve with the
frontal convection. There`s really zero point in trying to
speculate on details or specific timing, but it`s probably worth
being on the lookout for some kinks in the forecast as the front
rolls through.

Peering into early next week, there doesn`t appear to be much cold
advection behind the front to speak of, so this may well be a
situation where the increasingly powerful sun overwhelms any
potential cooling behind the front, and we actually end up with
warmer temperatures after the front passes. Indeed, that`s just
what I have in the forecast. My sweet, spring children, summer is
coming.
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jasons2k
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NWS Click Forecast:

Friday:
A chance of showers and thunderstorms, then showers and possibly a thunderstorm after 1pm. High near 77.

Chance of precipitation is 80%.
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DoctorMu
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Friday looking less impressive on 00Z GFS and CMC.
Cromagnum
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DoctorMu wrote: Tue Apr 20, 2021 1:55 am Friday looking less impressive on 00Z GFS and CMC.
Lucy been getting in a ton of practice lately.
Cpv17
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It looks like we will still have a severe weather threat Friday evening and we could still get 1-2” for a lot of southeast Texas but the best chance for any significant rain will be north and east of us. Looks like Louisiana into Arkansas and Mississippi will get the bulk of the rain.
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don
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This is looking to be more of a severe weather event more than flooding.Saying that, street flooding especially in urban areas is a possibility. It seems that models have come into agreement with the main surface low being further inland in the panhandle region. This decreases the amount of rainfall potential locally, but at the same time the surface low being further north should produce a more substantial warm sector,increasing the chance of severe weather here. SPC mentions that all modes of severe weather may be possible Friday.


day48prob.gif
Day 4-8 Convective Outlook
NWS Storm Prediction Center Norman OK
0358 AM CDT Tue Apr 20 2021

Valid 231200Z - 281200Z

...DISCUSSION...
Low-level moisture return appears likely across parts of the
southern Plains on Day 4/Friday as lee cyclogenesis occurs across
the southern High Plains ahead of an ejecting upper trough.
Medium-range guidance is now in reasonably good agreement that this
shortwave trough will eject eastward over the southern Plains on
Friday. Increasing low-level moisture to the east of a dryline/cold
front and steep mid-level lapse rates should support at least
moderate instability in a corridor across parts of OK and TX by
Friday afternoon. Strong mid-level flow and related deep-layer shear
appear favorable for organized severe storms, including supercells.
The low-level moisture should be somewhat greater in TX compared to
OK. As mid-level height falls/ascent overspread the warm sector,
robust convective development appears likely, with at least isolated
severe storms posing a threat for all severe hazards potentially
spreading eastward towards east/southeast TX and parts of the lower
MS Valley Friday evening/night in a strong low-level warm advection
regime.
Cromagnum
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Figures. Every year it happens where we go through a dry period so all the fence posts in the yard get loose with the ground cracking. Then we don't get rains to stiffen things up again, we get storms to blow them down.
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DoctorMu
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Cromagnum wrote: Tue Apr 20, 2021 10:37 am Figures. Every year it happens where we go through a dry period so all the fence posts in the yard get loose with the ground cracking. Then we don't get rains to stiffen things up again, we get storms to blow them down.
haha That's one thing I learned about fenceposts in the Brazos Valley. Steel posts with hardware and an 80 lb bag of cement + half a bucket of water. The fence may rot away but the posts remain!
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don
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Area Forecast Discussion
National Weather Service Houston/Galveston TX
317 PM CDT Tue Apr 20 2021


.LONG TERM [Wednesday Night Through Tuesday]...

Surface high pressure slides off to the east going into Wednesday
night bringing back a southeasterly flow to the region. Still
expecting another night of below normal temperatures though with
lows in the upper 40s/low 50s. With the onshore flow comes an
increase in moisture and an increase in temperatures! MaxT values
will reach the low-to-mid 70s on Thursday and the upper 70s on
Friday, the latter of which is seasonal for this time of year. With
steady WAA, PW values increasing to 1.2"-1.4" by Thursday afternoon,
and upper-level divergence, scattered rain showers are expected to
develop initially in the western portion of the CWA and spreading
eastward. The big story, which I`m sure you all are aware of, is
coming up for Friday/Saturday.

A shortwave trough out ahead of an upper-level trough will generate
lee cyclogenesis. Developing surface low pressure in north TX will
increase low-level moisture return with PW values increasing to
nearly 2" by Friday afternoon. Resultingly, widespread showers and
thunderstorms are expected Friday afternoon/Saturday morning. SPC
does have the entire CWA outlined in a 15% chance of severe weather
for Friday. With a strong inflow of low-level moisture and steep mid-
level lapse rates supporting sufficient instability, I don`t have
any reason to disagree with them. The probability of strong to
severe storms will exist ahead of (Friday afternoon) and along the
boundary of a "cold front" (Friday night/Saturday morning). This is
still three days out, so it`s too early to set things in stone. The
picture will become clearer once we get into the range of hi-res
model/CAM guidance.
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don
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Still have to get through Friday first but models are already showing another severe weather/ heavy rain setup this coming Tuesday/Wednesday fwiw.
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gfs_mslp_pcpn_frzn_scus_29.png
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don
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day3otlk_0730.gif
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0228 AM CDT Wed Apr 21 2021

Valid 231200Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY...

...SUMMARY...
Isolated to scattered severe thunderstorms should occur Friday into
Friday night from parts of the southern Plains into the lower
Mississippi Valley. Large to very large hail, damaging winds, and
isolated tornadoes all appear possible.

...Synopsis...
An upper trough should eject eastward from the Four Corners region
and Southwest across the southern/central Plains on Friday,
eventually reaching the lower/mid MS Valley by the end of the
period. An EML with associated steep mid-level lapse rates will
likely be present over much of OK/TX. A low-level jet should
continue to transport low-level moisture northward through the day
across the southern Plains, with a warm front potentially reaching
as far north as the OK/KS border vicinity. At the surface, a weak
low over the TX Panhandle Friday morning is forecast to develop
southeastward to the vicinity of southwestern OK/western north TX by
Friday evening while gradually deepening. A dryline should extend
southward from this low across portions of west/central TX. The
surface low should continue developing eastward to the lower MS
Valley late Friday into early Saturday morning, with a composite
dryline/cold front sweeping eastward across central/east TX. Rich
low-level moisture should advance northward ahead of the low/front
across much of LA and southern MS late in the period.

...Southern Plains...
Large-scale ascent associated with the shortwave trough should
overspread the warm sector across OK/TX by late Friday morning into
the early afternoon. As the cap erodes, robust convective
development appears likely along the eastward-mixing dryline. There
is still some uncertainty regarding the position of the dryline by
Friday afternoon across the southern Plains. Regardless, the
presence of rather steep mid-level lapse rates, increasing low-level
moisture, and diurnal heating should support moderate to locally
strong instability across the warm sector in OK/TX by Friday
afternoon. Deep-layer shear across this region appears strong enough
to support organized severe thunderstorms, including supercells.
Current expectations are for convection to initially develop along
or just east of the dryline, and subsequently move
east-northeastward. Large hail appears likely with this initial
activity, and isolated very large hail (2+ inch diameter) may also
occur. A veering/strengthening wind profile suggests some threat for
isolated tornadoes with any storms that can remain semi-discrete
through early Friday evening. Eventually, upscale growth may occur
along the cold front, with a damaging wind threat potentially
continuing into parts of east TX late Friday. Other isolated severe
storms may develop in the low-level warm advection regime in the
open warm sector along parts of the middle/upper TX Coast into east
TX through the day, posing a risk for all severe hazards.

...Sabine River Valley into the Lower Mississippi Valley...
A south-southwesterly low-level jet is forecast to strengthen
considerably from east TX into LA and southern MS Friday
evening/night. Surface-based convection appears increasingly
probable across this area as low-level moisture quickly increases.
The EML may advect eastward over this region, and NAM forecast
soundings show a favorable thermodynamic and kinematic environment
to support organized severe thunderstorms along/south of the warm
front. All hazards, including tornadoes, will be possible through
Friday night into early Saturday morning. The best severe threat may
be focused with the eastward-developing low-level jet, but
additional severe storms along the cold front may also occur. Even
with this seemingly favorable setup for severe thunderstorms, the
potential exists for afternoon storms with low-level warm advection
to slow the northward advance of the warm front and limit
instability. Once uncertainty regarding the evolution of these
afternoon storms is better resolved, greater severe probabilities
for tornadoes may need to be included for parts of the Sabine River
Valley vicinity.
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jasons2k
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It's a bit on the chilly side for me this morning, especially for late April.

No whining this summer that we didn't have a spring!
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don
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After looking at some of the 12z models i wouldn't be surprised at all to see most of the area locally to be under an enhanced risk Friday,and maybe even a moderate risk for severe weather in northeast Texas where shear will be greater.

There looks to be 2 main rounds of storms,the first with the warm front during the afternoon where there could be discreet supercells.And the second round coming with the cool front Friday night in the form of a squall line.
TXWeatherMan
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Feels great this morning, nice and refreshing.
Cpv17
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TXWeatherMan wrote: Wed Apr 21, 2021 12:17 pm Feels great this morning, nice and refreshing.
Enjoy it. Probably won’t feel it again till October.
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don
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I don't like the look of the 3k NAM showing numerous discreet supercells moving from the gulf, and supercell clusters over the area.
Attachments
Screenshot_2021-04-21 Models NAM 3km CONUS — Pivotal Weather.png
Screenshot_2021-04-21 Models NAM 3km CONUS — Pivotal Weather(1).png
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jasons2k
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NWS is already mentioning the "C" word for next week's system.

Hoping for some decent rains this Friday.
Cpv17
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jasons2k wrote: Wed Apr 21, 2021 5:35 pm NWS is already mentioning the "C" word for next week's system.

Hoping for some decent rains this Friday.
I’m not sure why. Models look decent for next week and there’s a pretty significant event that could happen after that to start off May.
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DoctorMu
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TXWeatherMan wrote: Wed Apr 21, 2021 12:17 pm Feels great this morning, nice and refreshing.
A/C off and sunny. Works for me.
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