Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 181949

Area Forecast Discussion
National Weather Service Birmingham AL
149 PM CST Sun Feb 18 2018

Afternoon Update.



Ridging aloft has migrated further east and is now centered to the
east of the Bahamas today. Global numerical models are in general
agreement in depicting the higher height region in an elongated
form extending from the western Atlantic Basin into the center of
the Gulf of Mexico. GOES-R low-level water vapor imagery along
with global modeling depicts a shortwave trough in the
southwesterly flow aloft over eastern Texas this afternoon. This
disturbance will continue to move northeast toward the state

At the surface, the cold front that moved south overnight
maintained momentum to reach the northern Gulf Coast early this
morning. A relatively quick-moving surface high pressure area
built into the Tennessee Valley region overnight and helped
discourage the surface front to the south from making much
northward progress. The surface high has quickly moved northeast
today and is now analyzed along the border between Ohio and West
Virginia. The position of the surface high this afternoon and its
low-level advection pattern has helped to reinforce and increase
the wedging effects experienced across portions of Georgia and our
eastern and northeast counties yesterday. A generally light
easterly flow is prevalent across many of our eastern and central
counties this afternoon and is helping to maintain cooler
temperatures and lower dew points compared to yesterday.

The 18/12Z BMX sounding contained a notable frontal inversion this
morning with last night`s cold front stalled along the northern
Gulf Coast. A fairly saturated layer was present from the surface
to the top of the inversion around 830 mb. A moist layer existed
above this point up to a subsidence inversion that was sampled
around 700 mb. A light northeast flow from 5-10 kts was observed
within the frontal inversion followed by a general unidirectional
zonal flow aloft. Speeds generally increased with height above the
inversion from around 25 kts around 850 mb to near 95 kts around
120 mb.


Today and Tonight.

Isentropic upglide from the stalled front along the northern Gulf
of Mexico is aiding in the developing of generally light showers
across much of our northern and central counties this afternoon.
This activity will continue to develop and move northeast,
providing the highest rain chances across our northern and central
counties this afternoon will become maximized across our far
northern and northeast counties later tonight as a shortwave
trough over eastern Texas approaches from the southwest.

Conditions closer to the surface this afternoon are cooler and
more stable compared to yesterday due to the weak wedging
influence from easterly flow. Temperatures are fairly uniform
generally north of the U.S. Highway 80 corridor with readings
ranging from the mid 60`s to low 60`s. With the frontal inversion
maintained, vertical mixing has been fairly minimal across much of
our forecast area except across our far southern and southeast
counties where cloud cover as been reduced, allowing for
temperatures to rise into the low to mid 70s at this hour.

Temperatures will remain the warmest across our far southern
counties through sunset as the stalled front gradually returns
northward as a warm front late this afternoon and tonight. With
vertical mixing maximized, some breezy conditions will be possible
across our southern counties until this evening when the boundary
layer will decouple. Some patchy fog will be possible across our
southern counties tonight as dew points rise as the warm front
passes northward. The fog potential will expand northward with the
northward advance of the warm front tonight. Have adjusted onset
of fog formation south in the grids and will mention fog in the
updated HWO product forthcoming. Lows tonight will range from the
lower 50`s north to the lower 60`s far south and southeast.


Monday through Sunday.

There are two main stories weather-wise for the extended - the above
normal temperatures and the rainfall. Synoptically, the warm front
has lifted north of the area by Monday morning, putting us well into
the warm sector ahead of the deepening trough and cold front. The
cold front is off to the northwest into Missouri, so any dynamic
forcing is limited for rain chances. This should leave Monday
mostly rain-free for Central AL, with only a slight chance in the
morning to give time for that warm front to lift out of here.
Strong ridging is in place to our south and east, leading to
deep/moist southerly flow.

First, the rain - we remain in the warm sector with really only the
possibility of isentropic lift to spark afternoon showers Tuesday
and Tuesday night. As we get into Wednesday, models are in good
agreement that the cold front has pushed into North MS and Western
TN. Lift along and ahead of this front will increase rain chances
across all of Central AL, with the rain likely north of the I-20
corridor Wednesday afternoon and night. The biggest question, and
the focus on all forecast uncertainty from Wednesday night through
Thursday is just how far that cold front gets. The GFS pushes it
through AL on Thursday before stalling it and lifting it northward
as another effective warm front. However, the ECMWF never really
brings it very far into Central AL (maybe to I-59 corridor) before
lifting it north Thursday morning (at least 24 hours sooner than
the GFS). I`ve been hedging my forecast closer to the ECMWF as it
has more support from the ensembles and the Canadian. This puts me
at around a 45-55% chance of rain for most areas north of the
I-85 corridor on Thursday for my forecast, but this still factors
in the GFS possibility. If the GFS trends closer to the EC in
coming days, I can decrease those PoPs more. Thunderstorms are
possible with this system as surface instability will be more than
enough to support thunderstorm development. However, shear values
are expected to remain too low to support any organize severe
threat. The warm front should be lifted north of our area by
Friday, regardless of which model verifies, so decreasing rain
chances overall for Friday and Saturday. I have kept some mention
for chance or rain in due to isentropic lift ahead of yet another
shortwave trough. That trough will push into Saturday night into
Sunday morning, increasing rain chances again for the weekend.
Have limited the PoPs to 45-50% on this trough due to the
possibility of fluctuations in timing.

Now, for the above normal temperatures - As most have noticed, it
has been unusually warm across Central AL the past few days. I
expect that to continue with the anomalous ridge off to our south
and east advecting warm/moist air into the area. Tuesday through
Thursday, the NAEFs percentiles are highlighting 99.5th to Max
climatological values for geopotential heights, temperatures,
specific humidity, and PWATS at around 3 standard deviations above
the mean(translation -it`s going to be warm and humid compared to
a typical mid to late February). Therefore, I have edged the high
temperatures up a few degrees, which will continue to put us in
potential record high territory Monday, Tuesday, and Wednesday. I
have included the records for our climate sites below this
discussion. Thursday`s high temps are somewhat uncertain and
dependent on how the GFS trends with that frontal passage. If it
trends more with the EC and keeps the front north of us, then we
could see more near-record highs again on Thursday. If the front
pushes through Central AL, we could see a temporary cool down on
the backside of the front. For locations along and south of the
I-85 corridor, there`s good chances the cold front never makes it
that far south, leading to continued above normal high temperatures
Thursday and Friday.



18Z TAF Discussion.

The cold front that moved south yesterday afternoon and evening
will return north as a warm front this afternoon. Highest coverage
of rain showers will affect our northern terminals this afternoon
with a reduced coverage at MGM/TOI. Tonight rain shower coverage
will decrease but lower CIG`s and some reduction in visibility is
expected with general IFR restrictions expected at most sites
through sunrise on Monday. Low level winds may become breezy at
times from the south this afternoon through sunset tonight with
the passage of the warm front.




Rain showers are expected this evening and into the overnight period
with the higher chances across our northern counties as a warm front
returns northward. Above normal humidity is expected to continue through
most of next week, limiting any fire weather concerns.



Unseasonably warm temperatures are expected to continue for the next
week or so. With forecast highs in the upper 70s and low 80s Monday
through Wednesday, several record highs will be in jeopardy. Here`s
a look at the current daily record highs for February 19th
through 21st.

Location            2/19        2/20        2/21
------------       ------      ------      ------
Birmingham        76(2011)*   78(1986)    78(1922)
Montgomery        83(1891)    80(2011)*   81(2011)
Tuscaloosa        77(2011)*   81(2014)    79(2005)
Anniston          76(2011)*   78(1986)*   75(2011)*
Calera            75(2017)*   76(2017)    75(2011)*
Troy              79(2014)    81(2014)    80(2005)

*Record was set in multiple years, but only the most recent is


Gadsden     53  75  57  79  61 /  40  20  10  20  30
Anniston    55  76  58  81  62 /  30  10  10  20  30
Birmingham  56  77  61  81  65 /  20  10  10  20  30
Tuscaloosa  57  78  61  81  64 /  20  10  10  20  30
Calera      57  78  61  81  64 /  20  10  10  20  30
Auburn      60  77  61  79  64 /  20  10  10  20  30
Montgomery  59  81  60  82  64 /  20  10  10  20  30
Troy        60  80  60  81  63 /  20  10  10  20  30




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