Post
by tireman4 » Thu Feb 25, 2021 10:39 am
000
FXUS64 KHGX 251152
AFDHGX
Area Forecast Discussion
National Weather Service Houston/Galveston TX
552 AM CST Thu Feb 25 2021
.AVIATION...
Guidance is very pessimistic with regards to ceilings and
visibility today and tonight but surface observations seem to tell
a slightly different story. ConShort and NBM sky grids show
ceilings below 800 feet for most of the day, while NAM and GFS
fcst soundings show MVFR ceilings transitioning to IFR tonight.
Surface observation have been trending from IFR to MVFR and now
VFR cigs. With moisture continuing to stream into the region under
a strong cap at 5000 feet, think there could be sufficient mixing
in this layer to allow for VFR ceilings before the inversion
lowers late in the day. Showers will stay mainly north of the
Houston terminals today with a slightly higher chance for precip
tonight as a weak cold front approaches. Intermittent periods of
sea fog will affect KGLS all day and again tonight. A mix of
IFR/MVFR and VFR ceilings will affect KGLS. 43
&&
.PREV DISCUSSION... /ISSUED 455 AM CST Thu Feb 25 2021/
SHORT TERM [Today Through Friday Night]...
A weak cold front has moved into SE TX and this boundary will serve
as a focus for showers today. At 3 AM, the front is along a Brenham
to Huntsville to Crockett line. The front will continue to sag south
and should make it around the I-69 corridor before stalling out.
Drier air has worked into the area behind the front but temperatures
have been slow to cool. This makes today`s temperature forecast a
bit of a challenge. The NAM looks to have a good handle on the
frontal movement but it still looks too cool with temperatures so
have leaned toward a MET/MAV/NBM blend. Models have also been
aggressive with high PoPs today (and last night) and precip has also
been slow to develop. PW values today are between 1.20 and 1.45
inches and the moist layer extends to around 800 mb with strong
capping in the 850-700 layer and a lot of dry air above the
inversion. Considering the moisture levels and the convergence along
the front, feel the potential is there for showers and will continue
with Chance PoPs for today. Sea fog has been affecting the coast
since last evening, but visibility has recently improved. Conditions
are favorable for dense fog as dew points in the upper 50s move over
water temps in the lower 50`s. Will maintain the NPW for dense fog
as the potential for additional sea fog remains high.
For tonight, the location of the front will directly impact MinT
values. NAM is once again the coldest of the models and again
blended the much colder NAM with the NBM and MAV guidance. The weak
boundary may actually retreat north a bit as weak low pressure
develops over Central TX and moves east and this feature should tug
the front back to the north a bit. Highest rain chances tonight
should be to the north of the sfc low and retreating boundary. There
looks like there is enough instability north of the front to
generate a few thunderstorms and have added isolated thunder to the
wx grids. South of the front, PW values lower to around 1.20 inches
with very dry air noted int he 850-500 mb layer. Will still carry
low PoPs over the south but most areas will likely remain dry.
Once the weak sfc low moving along the front pushes east of the
region another weak reinforcing boundary pushes into SE TX. This
makes the temperature forecast for Friday tenuous. Have once again
leaned toward blending the much cooler NAM with the MAV/NBM
guidance. The College Station area could remain in the lower 60`s
while the Houston area basks in the upper 70`s. The frontal location
will have a huge impact on MaxT values. PW values lower to around an
inch on Friday but with a weak boundary draped over the region, feel
there may be enough low level convergence to support a few showers.
Sea fog will remain a constant near the coast and Galveston should
expect intermittent periods of sea fog Friday and likely through
the weekend. 43
LONG TERM [Saturday Through Thursday]...
With onshore flow continuing to persist, PWAT values will steadily
rise and reach a peak near 1.50" by Saturday afternoon. Favorable
jet mechanics will be place for the northern half of the CWA along
with plenty of instability as CAPE values will range from 800-1000
J/kg and LI values from -4 to -2. LLJ develops over the northern
CWA going into Saturday night, so the environment will be suitable
for thunderstorms through at least Monday. With the persistent
onshore flow lasting through Monday, expecting temperatures to
remain rather mild with highs in the 70s and lows in the 50s/60s.
This is also indicative of the sea fog sticking around into early
next week as well.
Another cold front approaches the region on Monday morning, but
there are diverging ideas between the models on what occurs
following FROPA. Firstly, there is consensus on this front making
it off of the coast. PoPs are higher for Sunday night/Monday
since we will have low-level convergence along the frontal
boundary with PWAT values ranging from 1.50-1.75" still in the
area. Dew points will drop back into the 50s by Monday night and
should mark the end of this period of sea fog. Now let`s get to
the model differences. GFS/Canadian push the front off the coast
and have high pressure building in behind it for CAA lasting into
Wednesday. The ECMWF does much of the same but pushes the high
pressure eastward faster. This would bring warm, moist air back
for Wednesday, so this has a pretty big impact on temperatures and
rain chances. Added in low PoPs for Wednesday to account for
this. If water temperatures remain relatively cool, this would
also mean a return of the sea fog as well.
Thursday presents another major difference in the models as
the ECMWF/Canadian indicate lee cyclogenesis off the Rockies and
a strong cold front pushing through the area by Thursday morning
along with a pretty sizable positively-tilted shortwave. The GFS
also shows lee cyclogenesis, but it has a slower progression.
Still too far out to put a lot of weight into throwing in
significantly cooler temperatures, but have added PoPs in for
Thursday since rain looks likely along the frontal boundary. With
an unfavorable tilt in the shortwave trough and low instability,
not expecting thunderstorms for now. 26
MARINE...
Sea fog has developed in the Bays and nearshore waters and is
expected to persist throughout the day. The Dense Fog Advisory will
likely need to extended since the onshore flow will remain in place
with dew points holding steady in the low 60s. A weak cold front
will drift towards the coast but will stall inland later in the day.
There is a possibility that the front may extend far south enough to
give a brief break to the fog for portions of the Bays. The front
will then push back to the north by Friday morning, so it would be a
short-lived break at best. Onshore flow continues to persist into
Monday night when another cold front is expected to push off the
coast. The timing of this frontal passage differs from model to
model, but there is consensus on it pushing off the coast and ending
the period of sea fog. 26
&&
.PRELIMINARY POINT TEMPS/POPS...
College Station (CLL) 64 51 63 58 75 / 50 70 40 20 50
Houston (IAH) 73 64 75 63 75 / 40 40 30 20 30
Galveston (GLS) 66 60 68 61 67 / 30 30 20 20 20
&&
.HGX WATCHES/WARNINGS/ADVISORIES...
TX...Dense Fog Advisory until 9 AM CST this morning for the following
zones: Chambers...Coastal Galveston...Coastal Harris...
Galveston Island and Bolivar Peninsula.
GM...Dense Fog Advisory until noon CST today 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.
&&
$$