Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 220848

Area Forecast Discussion
National Weather Service Birmingham AL
348 AM CDT Fri Sep 22 2017

Today and Tonight.

Less fog is being reported so far this morning as low-level moisture
is confined to a very shallow layer near the surface. Most of the
fog will be near bodies of water, and in the northwest counties
where some rainfall occurred yesterday. There has been little change
to the upper-level pattern across the CONUS. An anomalously deep
trough remains over the western CONUS, a ridge axis remains from
Texas to southwest Ontario, and now post-tropical cyclone Jose
remains southeast of Cape Cod. A weak positively tilted mid and
upper-level trough continues to reach southwest from Jose to the
Gulf Coast. Within this trough an elongated upper low appears to be
drifting/shearing southwestward over Georgia and the Carolinas. A 50
kt north-northeasterly upper level jet streak is centered over
Alabama between the ridge and trough. The weak vort max that aided
yesterday`s convection should move to near Mobile by midday today.
Low-level ridging is producing low-level light easterly flow over
Central Alabama. Experimental GOES-R derived PWATs are a bit higher
than yesterday with values in the 1.4 to 1.5 inch range, highest in
the southern half of Central Alabama. Some drier air is noted over
far northern Alabama.

Forcing mechanisms for afternoon convection will be limited to
daytime heating, remnant outflow boundaries, and orographic lifting,
as is typical in summer. A couple outflow boundaries were present on
radar yesterday evening from earlier convection. One was moving
southeastward into the northwest counties, and another southward in
southwest Central Alabama. In the eastern half of the state, one was
moving southward towards the I-85 corridor, and another moved
northeastward out of Georgia towards the I-85 corridor and now may
be moving into the middle of the forecast area. Overall it`s
difficult to tell where these boundaries are currently, but that
will be something to watch in visible satellite imagery later
this morning. Only modest 1000-500 mb and 850-700 mb mean RH
values, as well as QG-subsidence associated with the ridge,
suggests coverage of afternoon showers/storms will remain only in
the scattered category today. Previous forecast showed slightly
higher PoPs along the I-85 corridor, and will stick with this idea
with slightly better moisture, and the possible boundaries. A
couple strong storms with small hail and gusty winds will be
possible with some DCAPE present. Also, 500 mb temperatures will
cool to around -10C in the eastern counties, allowing 700-500 mb
lapse rates to rise above 6 C/km, and mid-level flow will be
around 15 kts. Forecast microburst composite parameter values are
only in the low category, however. Convection should diminish
after sunset, with the possibility of patchy fog overnight near
bodies of water and where rain occurs.


Saturday through Thursday.

On Saturday, we have a rather busy looking upper air setup. A
piece of energy breaks off from a deteriorating Jose to over the
Northern Gulf of Mexico as Maria continues to trudge northward
out of the Caribbean toward the East Coast. At the same time a
deep trough digs across the Western US leaving a confined area of
weak upper ridging near the Mid MS River Valley into the Great
Lakes and NE US. Our result over C AL will be low diurnally
influenced pops with still status quo temperatures.

By Sunday, the upper energy from Jose fizzles further while the
low over the Northern Gulf retrogrades toward MS/LA, and Maria
moves further northward further constricting the weak ridging to
NE US while the West Coast system slowly moves over the Rockies.
On Monday we see Jose gone, absorbed into Maria. The system over
the Rockies starts to open up pulling the energy over the southern
states into it. Only weak flow remains over the Eastern US as
Maria monopolizes the flow. The result for C AL Sunday into Monday
will be only low pops south/southwest with better chances near
the Gulf Coast.

Tuesday into Wednesday, the main low associated with the Rockies
system moves northeast toward Manitoba. At the same time, ridging
fills in over the Gulf behind Maria and starts to creep around
into E Conus. During this time, we trend drier across Central
Alabama. By the latter half of the week, we see some divergence
in solutions as to weather we get a shortwave breaking off of the
main Rockies trough as the low pulls away across Canada and a
TX/Mexico ridge set up. Or we could get a somewhat flattening out
of the ridge but another low digging into the Northern Plains
from Canada. Either way, across C AL by the end of the forecast
Thursday, we remain on the dry side in advance of a slowly
approaching surface front across the US now that Maria is moving



06Z TAF Discussion.

Clear skies, light winds and low dewpoint depressions will result
in the potential for shallow ground fog again this morning.
Low-level moisture is confined to a very shallow layer right at
the surface, and guidance is less bullish on low visibilities.
Therefore, will only take visibilities down to MVFR (at all sites
except BHM) rather than IFR. Will have to monitor EET which was
the only site to receive a little bit of rain today. Any BR should
mix out quickly after sunrise. Summertime scattered showers and
storms are possible by afternoon, and added in a PROB30 mention at
all sites.




Isolated to scattered showers and storms are expected through
Saturday. Rain chances trend lower Sunday and Monday. Relative
humidity values only drop into the upper 40s and 50s. Transport
winds remain rather light which will keep dispersion values too.


Gadsden     87  66  86  65  85 /  20  20  20  10  10
Anniston    87  67  85  66  84 /  30  20  20  10  10
Birmingham  90  69  87  68  86 /  30  20  20  10  10
Tuscaloosa  92  70  89  69  87 /  30  20  20  10  20
Calera      89  69  86  68  86 /  30  20  20  10  10
Auburn      87  68  85  67  84 /  40  30  20  10  10
Montgomery  91  71  89  69  88 /  40  30  20  10  20
Troy        89  67  86  67  86 /  30  20  20  10  20





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