Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 261816

Area Forecast Discussion
National Weather Service Birmingham AL
116 PM CDT Sat May 26 2018

For 18Z Aviation.



Synoptic Analysis:
GOES-R moisture channel imagery along with global numerical model
depictions align well in depicting a trough aloft that extended
from Lake Michigan southward along the Mississippi River and over
the northwest Gulf of Mexico. Deep-layer ridging was depicted
just off the coast of the Carolinas and extended southeast over
the Bahamas and south Florida.

Toward the surface, high pressure was analyzed over the southwest
Atlantic basin that was nosing into our area from the east while
lower pressure was analyzed across New Mexico and Texas.

Subtropical Storm Alberto was analyzed by NHC located south of the
strait between the Yucatan Peninsula and western Cuba.

Mesoscale Analysis:
NEXRAD, GOES-R and surface analysis data reveals a weak low level
boundary across our northern counties associated with light shower
activity. This boundary extended from northern Marion County into
central Blount County and further southeast into south-central
Calhoun County and was lifting northward with time. Another
boundary was nearly stationary and extended from central Dallas
County northeast into western Chilton County with light showers in
the vicinity of this feature, mainly across Dallas County.

Additional boundaries may be present but identification was
hampered by abundant cloud cover.

Sounding Discussion:
The 26/12z BMX sounding continues to maintain a moist profile that
yielded a precipitable water (PWAT) value of 1.66 inches. There
exists a small subsidence inversion from around 940 mb upward to
near 650 mb. Winds were calm at the surface with a southwest flow
present above the surface to around 440 mb with speeds generally
from 10 to 20 knots. A southeast flow was measured higher aloft to
near 230 mb with speeds remaining between 10 and 20 knots. A more
westerly wind was detected above 230 mb with speeds from 10 to 15

The high PWAT values will again support a risk of localized
flooding in urbanized and poor-drainage areas while light winds
aloft will maintain slow storm motions that will only further
amplify the flash flooding risk. The lack of dry air in the
profile is helping to mitigate the wet microburst risk and today`s
risk is in the low category. The best potential for wet
microbursts exists across the western portion of the forecast
area. Modified observed soundings for this afternoon yields a
surface-based convective available potential energy (CAPE) value
over 2500 J/kg with a lifted index of -6. Robust updraft potential
may be realized depending on the how fast the cloud cover mixes
out through late morning and potential exists for a few stronger
storms, especially this afternoon.



Today and Tonight.

The ongoing forecast is in good condition with no changes needed,
short-term higher rain chances will exist across our northern
tier of counties due to showers associated with the northward
moving boundary. Showers may gradually increase in coverage in the
vicinity of the second boundary across our south-
central/southwest counties.


Revised Previous short-term discussion:

Convection today will once again be diurnal based with outflows
dominating development. The models are showing a mid level dry
slot moving from the Florida Panhandle and into Southeast Alabama
this afternoon. Based on this feature, lowest rain chances this
afternoon will be across the southeast counties, with likely pops
elsewhere. Activity will diminish in coverage and intensity during
the evening hours.



Sunday through Friday.

The main focus in the extended will be on Alberto and its
progression toward the area. Overall we see a west coast trough
swing through Plains on Monday into Tuesday. Until that trough can
make it closer to the region, we are going to remain on the outer
edge of the Bermuda High with the south/southeast flow continuing to
provide very rich moisture ahead of the arrival of Alberto. Rain
chances remain high for this time of the year, as the pattern will
not really change much at all until the trough passes through our
area by Wednesday night, finally getting rid of the tropical
airmass. With that said though this pattern change will place the
area in a northwest flow for Thursday through the weekend. This
could provide those MCS events each night, depending on what
develops to our north. More on that once we get through with

So after speaking with NHC and dissecting the model trends, we are
fairly confident on the overall timing of Alberto and rain chances
based on the track of the system. The consensus of the models take
the center near Mobile Bay Monday night and then slide it northward
along our western counties during the day Tuesday and possibly
exiting by Tuesday night. Look for the center of the low to then
become absorbed by the trough on Wednesday, resulting in front
moving westward through the area.

So to the specifics on impacts from Alberto. The initial band that is
usually 50 to 100 miles north of the center will be exiting the area
Sunday morning, with a second band of rain and storms moving into
the region Sunday afternoon/night and sliding north of the area by
Monday morning, just as models are expecting the center of Alberto
to begin to move into the coastline. The strongest winds will likely
make it into the southwestern areas between 7 and 10 pm Monday
night and only be around 25 to 35 mph with higher gusts (depression
strength). These winds will likely move north and then northeast
during the day Tuesday, all the while weakening as the system moves
north. Flooding and strong winds remain the primary threat but there
is a marginal chance at a brief spin up tornado in the southeast
Monday night into Tuesday as the system moves on shore with banding
on the northeast side of the low. As the system becomes more
organized will evaluate the need to add in brief tornadoes, but for
now will mainly focus on the rainfall and winds. On a whole we are
looking at 3 to 4 inches of rain area wide, with areas in the west
seeing 4 to 5 inches and locally higher totals.  Much more on this
system over the coming 24 to 48 hours.



18Z TAF Discussion.

Diurnal scattered showers and thunderstorms will once again move
through the area today. Included VCTS at most TAF sites through
the afternoon to account for uncertainty in timing and coverage.
Otherwise...any site that does have a thunderstorm or shower could
see MVFR visibilities and ceilings temporarily. Overnight...enough
low level moisture will be present for fog development...with LIFR
visibilities at any site that has rain this afternoon. Diurnal
showers and thunderstorms will be possible again Sunday.




A moist pattern is expected over the next seven days and should
result in daily rain chances. Heavy rainfall will occur with the
tropical moisture from Alberto on Monday and Tuesday. Patchy fog
and low clouds will be possible each morning particularly where
rainfall occurs during the prior afternoon and evening. No fire
weather concerns at this time.


Gadsden     67  81  66  81  68 /  40  60  70  60  60
Anniston    67  80  67  81  69 /  40  70  70  60  60
Birmingham  69  82  68  82  70 /  50  60  70  70  60
Tuscaloosa  69  83  69  84  70 /  40  60  70  80  80
Calera      67  80  68  82  69 /  50  70  70  70  70
Auburn      67  77  70  79  69 /  40  80  60  70  60
Montgomery  68  80  71  83  71 /  40  80  60  80  80
Troy        67  76  70  82  69 /  40  80  60  80  80




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