Re: MAY 2019: Active WX Week Ahead. Storms/Heavy Rains
Posted: Fri May 10, 2019 6:15 am
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Area Forecast Discussion
National Weather Service Houston/Galveston TX
540 AM CDT Fri May 10 2019
.DISCUSSION...
Key updates to the forecast:
- In general areawide 1-2 inches of rain has fallen in last 24
hours with several long swaths of 3-6 inches of rainfall.
- Overnight MCS has stabilized the boundary layer so the threat
for heavy rainfall today is lower. WPC now has slight risk of
excessive rainfall for flooding.
- Overall should have a break or lull in heavy rainfall threat
today due to the atmosphere taking some time to recover and
frontal boundary moving into the region. Still possible to get
another 0.5-1 inches of rain today.
- There is still a threat of heavy rainfall 06Z Saturday to 18Z
Saturday. Possible to get another 1-3 inches isolated 5 inches
during this time which could cause more flooding.
- Flash Flood watch will continue to account for a few storms
today but mainly for heavy rainfall expected tonight into
Saturday morning. At this point rather not drop the watch only
to put it back out for tonight.
.NEAR TERM [Through Today]...
Radar shows departing MCS out in the Gulf with storms trailing
back to the west and southwest along the Texas coast. Surface
analysis shows a cold front from basically south TX NE to Columbus
and Huntsville. Water vapor shows slow moving shortwave over the
area that helped initiate this most recent round of thunderstorms
and MCS. Another wave was located in the southern Rockies about to
pull int west Texas. A final wave was located west of the Baja
Peninsula rounding the base of an upper level low over California.
Overall analysis support trends for lower chances of showers and
thunderstorms than previously forecast due to the MCS stabilizing
the boundary layer making it less buoyant. Frontal boundary will
also have a similar effect, but with another jet streak expected
tonight, there could be some elevated storms that develop.
Forecast today keeps some slightly lower PoPs and rainfall amounts
than before to account for what the atmosphere is already doing.
Rainfall amounts today could still get up to an inch but there
looks to be a longer lull in activity than previously thought.
This may give bayous, creeks and rivers a chance to decrease in
levels as well as allow for streets and underpasses to drain.
Since there is still a chance of a few storms and we are expecting
another round of storms that have a higher potential to cause
heavy rainfall tonight into Saturday, we will continue the flash
flood watch as it is.
.SHORT TERM [Tonight Through Saturday]...
The jet streak of concern for our forecast is now located back
west of the Baja Peninsula. This jet streak should move across
Mexico today and reach south TX and southeast TX Saturday morning.
Synoptic models spread divergence aloft over the area with large
scale ascent. Ageostrophic circulation enhances the low level jet
tonight into Saturday morning with a 20-30kt 850mb LLJ. This
allows for much deeper moisture to return the coastal areas with
precipitable water values increasing to around 1.8 to 2 inches.
GFS has been focusing this higher moisture plume off the coast
into southwest LA. The ECMWF has a more southerly directed LLJ and
develops deep moisture farther inland. I think these scenarios
largely hinge on where the cold front boundary stalls. GFS looks
to push this front closer to the coast where ECMWF is a bit more
inland. This boundary will again serve as a focus for more
convection especially if precipitable water values of 2 inches
materialize inland. ECMWF seems to be more robust with QPF than
the other synoptic models but the WRF NMM/ARW forecasts also have
high QPF focused along the coast and this boundary. HREF also has
a strong signal for rainfall of 1-3 inches mainly south of I-10.
These rainfall totals look on track with other models so will go
with rainfall of 1 to 3 inches with isolated 5 inches as an upper
bound. Given the overall pattern of a jet streak, front, moderate
LLJ and an increase in northward moisture transport, I think the
flash flood watch looks on track.
As for impacts, we are already seeing rapid rises on bayous,
creeks, rivers, some of which in flood along with widespread
street flooding. Expected impacts may be much of the same for
tonight into Saturday. However today, the main message is to stay
out of flooded areas, turn around do not drown, do not go around
barricades and closed roads. Tonight into Saturday could be
another stay put situation as additional rainfall could cause
bayous to go out of banks quickly and low lying
areas/underpasses/streets will flood again.
.LONG TERM [Sunday Through Friday]...
Storm system should move out of the area Sunday with the cold
front off the coast. The upper level low over California finally
starts to move east and reaches west TX on Monday. Synoptic models
have some significant differences in timing of the next rainfall.
GFS shows more rainfall on Monday with precipitable water values
getting up to 1.7 to 2 inches over the area. Meanwhile the
ECMWF/CMC are delayed in the return of rainfall with the bulk of
the activity in these models Tuesday into Wednesday. Forecast
leans more on the ECMWF/CMC with higher rain chances more on
Wednesday than on Monday or Tuesday. We will still keep 30 to 40
PoPs for Monday and Tuesday to cover the uncertainty in timing.
After Wednesday we might catch a break from the rainfall and dry
out a bit for Thursday and Friday.
&&
.MARINE...
Small craft advisory will go through the morning hours with
strong winds due to the thunderstorm activity with even some gusts
to gale force at times. Otherwise winds and seas should decrease
until the next round of thunderstorm activity expected tonight
into Saturday morning. Offshore winds should develop Sunday with
easterly winds on Monday. This will allow for seas to decrease to
start next week.
&&
&&
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 68 62 75 60 78 / 40 70 70 10 10
Houston (IAH) 76 68 79 64 81 / 60 70 80 10 10
Galveston (GLS) 78 74 80 70 80 / 90 80 80 20 20
&&
.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Flash Flood Watch through Saturday evening for the following
zones: Austin...Brazoria
Islands...Brazos...Burleson...Chambers...Coastal
Brazoria...Coastal Galveston...Coastal Harris...Coastal
Jackson...Coastal Matagorda...Colorado...Fort
Bend...Galveston Island and Bolivar
Peninsula...Grimes...Houston...Inland Brazoria...Inland
Galveston...Inland Harris...Inland Jackson...Inland
Matagorda...Madison...Matagorda
Islands...Montgomery...Northern Liberty...Polk...San
Jacinto...Southern
Liberty...Trinity...Walker...Waller...Washington...Wharton.
GM...Small Craft Advisory until 11 AM CDT this morning for the
following zones: Coastal waters from Freeport to Matagorda
Ship Channel TX out 20 NM...Coastal waters from High Island
to Freeport TX out 20 NM...Galveston Bay...Matagorda
Bay...Waters from Freeport to Matagorda Ship Channel TX
from 20 to 60 NM...Waters from High Island to Freeport TX
from 20 to 60 NM.
&&
$$
DISCUSSION...Overpeck
NEAR TERM...Overpeck
SHORT TERM...Overpeck
LONG TERM...Overpeck
AVIATION...Luchs