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Area Forecast Discussion (AFD)

Issued by Mobile/Pensacola (MOB)

FXUS64 KMOB 281114 AAA

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
614 AM CDT Fri Jul 28 2017

.DISCUSSION...Updated for latest aviation discussion below.


12Z issuance...VFR conditions expected today into this evening
over the area. A weak front sagging south towards the area will
bring increasing coverage of mid and low level clouds from the
north. Through sunrise Saturday, am expecting any MVFR or lower 
CIGs/VISBYS to remain well north of the I-10 corridor.



.PREV DISCUSSION... /issued 441 AM CDT Fri Jul 28 2017/ 

NEAR TERM /Now Through Friday night/...Looking at the current
picture over the Conus, a shallow upper trough has developed 
over the East Coast, with an upper ridge stretching sw over the 
Southern Plains and western Gulf from the Desert Southwest. A 
strong shortwave system is digging south across the Great Lakes 
into the upper trough. A weakly organized surface high has 
developed over the western Gulf. The most notable item sticking 
out in the current analysis is a slug over very dry air stretching
from over southern LA/MS northeast across central AL. This is 
notable due to the fact that it seems that the forecast area has 
been living under a tropical airmass the last month. 00Z sounding 
from KJAN was a dry 1.38", with KLIX at 1.49", KBMX at 1.44" and 
KTLH with 1.56"; all below seasonal. 

As the Great Lakes system dives southeast today through tonight, it 
pushes a surface front south across the Tennessee River Valley, then 
Southeast. Today, it will start to push a moist airmass south across 
northern-most regions of the forecast area, mainly this afternoon. 
Until then, the very dry airmass combined with some upper subsidence 
from the upper ridge just to the west bring temps in the mid to 
upper 90s. Enough moisture remains in place to allow Heat Indices to 
rise into the 90 to 103 degree range today. Am expecting isolated 
shra/tsra to push develop/push over northern-most regions of the 
forecast area north of Highway 84, but most of the forecast area is 
expected to remain rain-free today. 

Tonight, as the shortwave system pushes south across the Eastern 
Conus, a surface boundary along with significantly more moisture 
pushes south across the Southeast from the Tennessee River Valley, 
increasing the rain coverage as the night progresses. Best chance of 
rain continues to be north of Highway 84, closer to the surface 
front along with any outflows from previous activity. Guidance is 
consistent in advertising the coast remaining generally rain-free 
until after midnight, and see no reason to argue at this point. Have 
put in a southward progression of the rain through the night in the 
grids to reflect this. Have went around to a bit above seasonal with 
temps, with increasing cloud cover from the activity to the north 
and northwest cutting off the chances of radiational cooling. Add in 
a bit of initial subsidence ( the easter trough deepens overnight, 
pushing the Southern Plains ridge back west), and feel around to a 
bit above lows are warranted. Lows generally in the low to mid 70s, 
mid to upper close to the coast. /16

SHORT TERM /Saturday Through Sunday night/...The upper level
trough with embedded closed low pressure east of the Mississippi
River will continue to dig southward through the short term with 
the base reaching the southern tip of Florida. Meanwhile, an
associated weak cold front approaching the area from the north 
will move into the northern portion of the forecast area Saturday 
morning, and move slowly through the forecast area throughout the
day Saturday and Saturday evening.

Precipitable water values will increase to between 2.4 to 2.7 
inches ahead of the front by Saturday morning, with MLCapes values
reaching between 1200 to 1500 J/KG by late Saturday afternoon. 
SBCapes will naturally be high and range from 1700 to 2200 J/KG,
possible as high as 2700 J/KG just ahead of the front. Shear in 
the SFC-3KM range however will remain very low (below 100 m2/s2).

The high Cape values will result in a marginal risk of severe
thunderstorms across the forecast area on Saturday, favoring a 
few wet microbursts with damaging winds. The shear profile 
otherwise will be unfavorable for any organized severe threat.

Heavy rainfall will also occur with this line of storms with 
additional widespread amounts along and south of highway 84 
ranging from 0.45 to 0.70 inches, with locally higher amounts 
between 2 to 3 inches under areas where more prolonged stronger 
storms persist.

High temperatures over the weekend will be slightly below normal 
ranging from the upper 80s to around 90 degrees. Low temperatures 
each night will be in the upper 60s to lower 70s, except for mid 
70s along the immediate coastal sections. /22

LONG TERM /Monday Through Thursday/...A weak surface ridge of
high pressure will remain across the southeast states Monday and
Tuesday while the stalled surface boundary over the northern gulf
dissipates. The surface ridge will then weaken through midweek as
a surface trough builds along the northern gulf coast from the
west. Meanwhile, the upper level trough over the western Atlantic
just off the eastern seaboard will continue to move eastward,
followed by a larger broader trough digging southward across the
eastern conus.

Isolated showers and thunderstorms are expected to return to the
southwest portion of the forecast area and along the coast on 
Monday, and across the southern one-third of the forecast area on
Tuesday. Precipitation chances will then increase Tuesday night
through Thursday as the two aforementioned features take affect
over the region.

High temperatures Monday through Thursday will be slightly below
normal ranging from the upper 80s to around 90 degrees. Low
temperatures each night will be in the upper 60s to lower 70s,
except for mid 70s along the immediate coastal sections. /22

MARINE...A weak surface front will sag south across the forecast 
area Saturday into Saturday evening, before stalling over the 
northern Gulf early Sunday. This will shift the light, general 
westerly flow to offshore. With the boundary approaching the 
northern Gulf coast late tonight into Saturday, a tightening of the 
pressure gradient along the coast will bring moderate to at times 
strong westerly winds to unprotected coastal waters late tonight 
into Sunday. An SCY may be needed if winds/seas rise any higher, so 
will have to monitor things. The boundary weakens through the 
weekend, though with more shortwave energy diving into the eastern 
upper trough late in the weekend, guidance is developing a weakly 
organized surface low over the northern Gulf Sunday night into 
Monday. Current guidance is advertising the northeast Gulf, though 
position has been inconsistent in the guidance the last few days. 
The current placement will bring a light to moderate northeast to 
easterly flow to the marine areas for the beginning of the week.




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