Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Birmingham Alabama
600 PM CST Sat Nov 18 2017
for 00z aviation.
Several things to talk about late this afternoon. A strong cold
front was rapidly approaching central Alabama. The front has
already entered northern Mississippi. It appears to be moving
around 50 mph east southeast. This puts the front entering
northwest areas just before 6 PM, near the I-59 corridor at 8 PM,
Montgomery to Wedowee around 10 PM, and exiting the area by around
midnight. This speed may slow a tad and these times may need to
backed up an hour or so.
Strong to marginally severe storms...a line of thunderstorms has
developed along the leading edge of the front. Some pretty decent
reflectivity gradients, bows and lewps are beginning to show up.
At this time, our threat area of the northwest quadrant of
central Alabama looks good. Surface based cape has struggled but
made it near 300, as dew points have risen into the mid 60s and
temperatures jumped into the mid and upper 70s in places. Plenty
of shear for some storm organization but we originally had doubts
about the buoyancy. Too much shear and not enough buoyancy tips
the updrafts over. At any rate, expect a few marginally severe
storms in the northwest quadrant between 530 PM and 8 PM. The
threat will be wind gusts in the 40-60 mph range. The storms
should weaken as they progress eastward.
Wind gusts outside storms...a strong pressure gradient has set
over the deep south. Winds above the surface have also increased.
Daytime heating and mixing of the atmosphere has lead to wind
gusts. These wind gusts may reach 35 mph or more roughly north of
a line from Aliceville to Wedowee, where a Wind Advisory is
currently in place. Elsewhere, gusty winds are expected but the
winds will be slightly less. The strong winds will begin to
subside west to east starting around 10 PM and diminishing by 3
It looks like a good bet that large part of central Alabama will
experience freezing conditions as high pressure settles in Monday
morning. At this time, lows will range from near 25 to near 33
degrees. A freeze watch has been issued for all of central
Previous short-term discussion:
added Wind Advisory due to slightly increased winds above the
surface and potential mixing into this layer below a subsidence
inversion. A few wind gusts possible in the 35 mph range north
into the overnight hours. The winds will subside behind the front
west to east after 10 PM. This threat resides mainly across the
north, north of a line from Aliceville to Wedowee. Still breezy
south and will monitor conditions into the evening hours there.
There is a small risk of a strong to severe storm this afternoon
and evening. A somewhat typical high shear and Low Cape
environment expected, with limited overall moisture and buoyancy.
Will mention a few wind gusts into the 60 mph range of some storms
can get going. This threat area is roughly north of a line from
Aliceville to Birmingham to Oneonta.
Previous short-term discussion:today and tonight.
A wide range of temperatures early this morning with lower 60s
across West Alabama and mid 40s across east Alabama. Warm air
advection has increased significantly for areas along and west of
Interstate 65 where southerly surface winds have been running 5 to
10 mph. Low clouds have also developed for areas across east
Mississippi and western Alabama. No rain was currently indicated
across the region as the air mass is still relatively dry with
precipitable water values around one inch. A few showers may develop
by late morning across northwest Alabama due to strong warm air
advection and increasing low level moisture. Most of the shower and
thunderstorm activity will be confined to near the surface front due
to a strong pre-frontal upper level inversion. The surface cold
front will enter Marion and Lamar counties late this afternoon and
quickly track southeast, reaching Troy near midnight tonight.
Thunderstorm activity looks very limited ahead of the surface front
due to weak mid level lapse rates and cape values struggling to
reach 500 j/kg. Any thunderstorms that may develop will likely be
north of I-59. The rain will end within one or two hours after
fropa, so rainfall amounts will be mostly under one-quarter of an
Sunday through Saturday.
The cold front is well to our southeast by Sunday morning, so expect
clear skies as high pressure builds in with the colder air mass. The
center of the surface high slides eastward through the arklatex and
into the Tennessee River valley Sunday night into Monday morning. Models
are in pretty good agreement with the center moving across north-
central Alabama by sunrise Monday morning. This will lead to clear skies
and calm winds across the area, allowing for very efficient
radiational cooling. Expect much of the area to drop to near or
below freezing early Monday morning with the coldest temperatures
(mid to upper 20s) in the northern counties. With increasing model
agreement in the setup, I will continue mention cold weather impacts
in the severe weather potential statement and increase the confidence. After discussions with
neighboring offices, will hold off any freeze watch or warning
products for now, as we're still 48 hours out.
The high pressure shifts eastward towards the Carolinas during the
day Monday. Another longwave trough deepens across the eastern
plains Monday night into Tuesday. Both the GFS and European model (ecmwf) pick up on
a weak impulse ejecting out of the base of the trough Tuesday
morning. This impulse moves through central Alabama fairly quickly, and
without much moisture return, i'm not convince we'll see any rain
activity out of it. However, have included at least a slight chance
of showers south and east of the I-85 corridor Tuesday just in case.
As we get into the second half of the week, the forecast becomes
somewhat of a headache. Overall, the synoptic setup is a highly
amplified pattern with a deep trough over the eastern US and a ridge
over the western US. An impulse moves through the trough into the
southeast US on Wednesday, and the models have not handled the
evolution of the trough and this impulse very well. The GFS, which
had been showing a phased trough moving through now shows a cutoff
low developing in the northern Gulf of Mexico and sliding eastward
into southern Georgia. The ECMWF, which had been showing something
similar now shows a phased trough moving through with high pressure
quickly building in behind. Now, remember that most of this week
we're under northerly or northeasterly flow, which limits our
moisture return. Under normal circumstances, this would make ME lean
on a dry forecast. The GFS solution, however, has a fairly strong
surface low developing in the eastern Gulf of Mexico, which as it
slides into southern GA, could lead to enough wrap-around moisture
to cause light shower activity in our southeastern counties.
Essentially, the forecast comes down to (1.) If (or where) a low
develops along the coast and (2.) How strong the low is if it forms.
If a surface low does not form, or if it is weak, expect mostly dry
conditions through the Holiday weekend. If a surface low does form
and can pull in enough moisture, expect some light rain showers
Wednesday night through Thursday before high pressure again builds
in Friday into Saturday leading to a dry start to the weekend. For
my forecast this morning, I have edged drier given the lack of
substantial moisture return, but kept a slight chance of showers
south and east of the I-85 corridor on Thanksgiving just in case.
Confidence remains low in the forecast from Wednesday night through
Saturday, so expect changes as models hopefully become more
00z taf discussion.
Some MVFR cigs and vsbys will be possible with thunderstorms and rain along a cold
front that will move across central Alabama this evening. Gusty
winds will be possible with the storms and for a couple of hours
behind the storms as winds shift with front. Storms along line are
not as strong to south and do not have winds quite as high for
mgm/toi with passage. Skies clear out during the late night hours
behind front going VFR before sunrise. Winds will be northwest and
gusty at times during the day on Sunday with tight surface pressure
gradients behind the front across the area.
Windy conditions will continue ahead of a cold front. The front
passes through tonight with winds remaining gusty behind the
front. Winds will subside from west to east between 10 PM and 3
am. Rain chances are high areawide, but the rain amounts will be a
quarter inch or less and the rain chances will only last 2-3
hours in any one location. Dry and colder air moves in behind the
front Sunday and Monday. Relative humidity values drop into the
30s Monday afternoon.
Preliminary point temps/pops...
Gadsden 39 50 27 57 35 / 90 0 0 0 0
Anniston 40 51 28 59 38 / 90 0 0 0 0
Birmingham 42 51 31 58 40 / 90 0 0 0 0
Tuscaloosa 41 54 29 59 38 / 90 0 0 0 0
Calera 40 53 29 58 38 / 90 0 0 0 0
Auburn 43 53 32 59 44 / 70 0 0 0 0
Montgomery 44 56 32 60 41 / 70 0 0 0 0
Troy 45 57 32 61 42 / 60 0 0 0 0
freeze watch from Sunday evening through Monday morning for the
following counties: Autauga...Barbour...Bibb...Blount...
Wind Advisory until 3 am CST Sunday for the following counties: