Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 210852

Area Forecast Discussion
National Weather Service Birmingham AL
352 AM CDT Thu Sep 21 2017

Today and Tonight.

Upper 60s to low 70s dewpoints and late September radiational
cooling have led to areas of ground fog developing once again across
the area, especially near bodies of water. Earlier cirrus continues
to dissipate, and winds are calm. Visibilities aren`t as low as last
night so far, as it did not rain yesterday in most places, and the
near-surface moisture depth is shallower. Therefore, have held off
on a dense fog advisory but will continue to monitor; a mention of
visibilities below one mile is included in the HWO. Fog should mix
out soon after sunrise.

Tropical Storm Jose remains trapped off of the Massachusetts coast
by high pressure to its north. This is helping to result in a blocky
pattern over the eastern CONUS. A weak upper low is located over the
Carolinas, with a positively tilted upper trough axis located from
Jose to this upper low and southwestward into southeast Alabama. A
wedge of dry air is located parallel to this trough axis with
experimental GOES-16 derived PWAT values of 1.2 inches, while PWATs
have rebounded to around 1.4 inches northwest of this area.
Upstream, another weak quasi-stationary upper low is located over
Louisiana, its motion blocked by the trough axis. A mid-level ridge
axis extends from Texas northeastward towards Michigan, downstream
of a deep positively tilted trough over the western CONUS. North-
northeasterly flow will be present over Central Alabama between the
ridge and the East Coast trough. At the surface, an occluded low is
located over northern Manitoba/Saskatchewan with a cold front
located over the Midwest back into the Plains. This front will not
make much of any progress today. A surface ridge is located over the
eastern CONUS.

The best chances of scattered showers/storms today will be across
the northern counties due to slightly better deep layer moisture and
a weak vort max in the north-northeasterly flow aloft. A north-
northeasterly jet streak moving down the back side of the trough
will also contribute some upper level divergence. Some CAMs try to
generate a small cluster of storms around sunrise over Middle
Tennessee due to the vort max, but there are no signs of this
happening yet. It would be unlikely to hold together and make it
into our area but could create an outflow boundary. With 1000-500 mb
mean RH values and 850-700 mb mean RH values not being anything to
write home about, will cap PoPs at 30-35%. A couple storms could
produce gusty winds due to some dry air aloft. A separate area of
scattered convection could also develop over our far southeast
counties due to the trough axis. In between these two features, the
lingering wedge of dry air could largely suppress convection. But
will keep an isolated mention in as the area of dry air will be
slowly moistening, and low-level moisture remains seasonably high.
Isolated showers/storms could linger into the early evening before
dissipating. Persistence suggests patchy fog will be possible again
tonight especially near bodies of water.


Friday through Wednesday.

We start off on Friday with a tale of three upper lows. First, we
have Jose which continues to churn off the NE US coast with
troughing extending outward to the SW into the NE Gulf of Mexico.
Next, there is Maria moving NW toward the Atlantic Seaboard from
the Caribbean. Lastly, we have a large scale trough over the
Rockies. As Jose weakens and Maria gets closer, some of the energy
will break off to the SW Saturday and spin over the Deep South.
By Sunday, the upper energy from Jose is absorbed into Maria
while the low over the South opens up/weakens and with the flow
getting pinched by both Maria and the slowly approaching W Conus
trough with a weak upper ridging building into LA/MS/AL Tuesday
into Wednesday.

At the surface, we continue with easterly flow across Alabama for
Friday and into the first part of next week around a weak
elongated E Conus ridging centered across the NE US. Further to
the west we have a surface front extending from MN southwestward
to SD and into the Rockies, with its progress eastward impeded by
Jose and Maria. For the first part of next week with Jose
dissipated and Maria making progress northward, we should see the
W Conus surface front making gradual progress across the Plains
and toward the Great Lakes/MS River/Texas region by the middle of
next week.

How does the above equate to us in Central Alabama? We will see
low rain chances Friday into Saturday with disturbances associated
with the weak elongated troughing from Jose. For Sunday and
Monday, the N Gulf low will spurn some showers with the greatest
chances near the Gulf Coast, but a few could scrape our southwest
counties during the daytime hours mainly. For Tuesday into
Wednesday, weak overall ridging and weak to northerly low level
flow around Maria should suppress convection. Temperatures will
vary little from day to day with no big changes in airmass with
lows and highs running a couple of degrees above normal for this
time of year.



06Z TAF Discussion.

Muggy dewpoints, late September radiational cooling, and nearby
moisture sources (bodies of water) will result in another round
of shallow ground fog at all sites except BHM. Fog may not be
quite as dense as last night as it did not rain on Wednesday and
the near-surface moisture depth is shallower, but still expect vis
to drop in the IFR category at these sites. Fog should quickly mix
out after sunrise. Added in a PROB30 for afternoon scattered
thunderstorms near BHM/TCL where slightly better moisture will be
along with a weak disturbance in northeast flow aloft, after also
consulting latest CAM runs.




Isolated to scattered showers and storms are expected today.
A similar pattern will continue for the next couple of days. Rain
chances trend lower as we go into next week. There are no fire
weather concerns.


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





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