Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 172010

Area Forecast Discussion
National Weather Service Birmingham AL
148 PM CST Sat Feb 17 2018

Afternoon Update.



GOES-R Water vapor and global numerical models both depict the
center of a broad mid layer ridge remaining over the central Gulf
of Mexico. The ridge has become more elongated zonally over the
past 48 hours, allowing our forecast area to experience a series
of shortwave disturbances in the general southwest flow to the
north of the ridge.

Toward the surface, a cold front that moved into the state
yesterday afternoon stalled over our southern counties early this
morning and since has retreated northward as a warm front and is
now located over the Tennessee Valley region. A very weak
influence by surface high pressure off the coast of New England
down the Appalachians resulted in a weak easterly flow across our
east-central and northeastern counties this morning into early
afternoon that resulted in lower dew points and slightly cooler
temperatures. This influence has largely been mitigated at this
hour by vertical mixing.

A pronounced frontal inversion was present on the 17/12Z BMX
sounding this morning up to near 935 mb followed by a near
saturated region up to near 745 mb with a strong subsidence
inversion that extended further aloft to just beyond 350 mb. A
light low level easterly flow quickly veered to the south around
970 mb and increased to near 10 kts followed by a unidrectional
southwest flow from around 895 mb that extended further aloft to
near 170 mb. Highest measured winds were around 110 kts from zonal
flow around 150 mb.



Today and Tonight.

A shortwave trough in the southwest flow aloft is supporting the
southeast advance of the cold front that returned north this
morning. The front is approaching our far northwest counties at
this hour and will result in higher rain chances expanding
southeast across our forecast area later this afternoon and into

Similar to yesterday, temperatures will fall at least 10 degrees
with the frontal passage with a wind shift from the north-
northwest upon arrival. Ahead of the front, vertical mixing has
helped to reduce cloud cover and is promoting breezy conditions at
times as regional VWP data shows near 50 kt retrievals around 5000
ft. Breezy conditions will remain possible through sunset for
areas ahead of the cold front before vertical mixing subsides.

The southwest flow pattern aloft with limited amplitude will
continue to keep the cold front from clearing our forecast area
tonight with the boundary likely decelerating this evening and
stalling across the southern third of our forecast area overnight.
The front is expected to again lift northward early on Sunday as a
warm front.

A pronounced thermal gradient will again be present across our
forecast area overnight with lows in the low to mid 40`s tonight
with colder readings across our northwest counties. Meanwhile,
readings will only fall to the lower 60`s across our far southeast
counties as the front stalls to the north of these locations.


Sunday through Saturday.

The back and forth battle between a ridge to our southeast and
the troughing to our north continues through the next 7 days. A
couple of shortwaves move through pushing cold fronts into the
area. Each will stall somewhere in South-Central or Southern AL
before lifting back northward as an effective warm front thanks to
the ridging. This will lead to a continuous cycle of rain chances
across Central AL.

First up on Sunday is the cold front that pushed into South AL
earlier this weekend. It will be lifted back northward as a warm
front increasing rain chances Sunday afternoon across much of the
area ahead of the deepening shortwave trough to our northwest. We
could see a break in the rain activity Monday and Tuesday as we
remain in the warm sector with the cold front off to our west
moving through the Ozarks.

The next cold front is expected to push into the area around mid-
week, but the exact timing is uncertain. The GFS is at least 24+
hours more progressive with the front and best rain chances getting
into Central AL compared to the ECMWF. Both have the front
essentially stalling Wednesday night into Thursday, but the EC has
it slowing/stalling just to our north and west while the GFS has it
stalling in Central to South AL.

Models keep this front stalled through Thursday night/Friday morning
before it`s lifted back north as an effective warm front, though
just how quickly it gets lifted is still questionable. Overall,
model differences in timing of this next front on Wednesday and
where it slows down/stalls makes the forecast for the second half of
the week difficult. I`ve carried 60-70% chance of rain for Wednesday
north of the I-20 corridor, but have decreased the PoPs to 50% on
Thursday simply due to the uncertainty of where that front will be
located. I have also included mention for thunderstorms Wednesday
and Thursday due to instability that will be in place across the
area. 0-6km shear values are 20-25kts - too low to support any
organized severe threat, therefore I will not add mention in the
HWO. Thursday night into Friday, I`ve kept lower PoPs (25-30%)
leaning more towards the ECMWF solution of pushing the warm front
north of the area and keeping us mostly rain-free. The 25-30% is
accounting for the potential of the GFS solution for rain
continuing through the end of the week.

By this coming weekend, models hint at yet another shortwave
moving through the trough, and pushing another cold front into the
area, but the timing is just beyond the 7 day forecast, so have
just included chance PoPs in for Saturday.



18Z TAF Discussion.

A surface warm front lifted north of our forecast area this
morning resulting in lower rain chances through early afternoon.
The front is expected to stall over southern Tennessee over the
next few hours and return southward as a cold front this evening.
Expect the coverage of showers to increase through the afternoon
at our northern sites and become more widespread this evening. The
southern terminals will see increased rain chances this evening
and lasting into early Sunday morning with the arrival of the
front across our southern counties.

Low level winds will be breezy ahead of the front this afternoon
then will decrease this evening with the front nearby. Some
reductions to VIS will be possible across our southern sites early
on Sunday with the front in the vicinity.




Rain showers are forecast tonight through Sunday as a front arrives
from the northwest and stalls across our southern counties
overnight. Above normal dewpoints will preclude any fire weather
concerns through at least the next week.


Gadsden     45  64  54  74  57 /  90  40  40  20  10
Anniston    48  65  56  76  58 /  90  40  30  20  10
Birmingham  47  64  58  77  60 /  90  40  20  20  10
Tuscaloosa  49  64  58  77  61 /  90  40  10  20  10
Calera      49  65  58  78  60 /  90  30  20  20  10
Auburn      55  67  59  77  60 /  60  30  10  10  10
Montgomery  55  71  59  81  60 /  70  20  10  10  10
Troy        60  73  58  80  60 /  30  10  10  10  10




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