832 
FXUS64 KMOB 181146
AFDMOB

Area Forecast Discussion
National Weather Service Mobile AL
646 AM CDT Wed Aug 18 2021

.AVIATION...
12Z issuance...VFR is generally prevalent across our forecast area
early this morning, with the exception of a few small patches of
MVFR to IFR ceilings and visibility reductions across portions of
south central/southwest AL and southeast MS. VFR continues through
the period, except for localized reduced ceilings/visibility
values near isolated to scattered SHRA/TSRA that will develop
northward across the area late this morning into this afternoon/ 
early evening. A few MVFR ceilings and/or MVFR to IFR visibility 
reductions will also be possible again late tonight into early 
Thursday morning across parts of interior southwest/south central
AL and southeast MS. Southeasterly to southerly winds will increase
to 5-10 knots today before becoming light by early this evening. /21

&&

.PREV DISCUSSION... /issued 434 AM CDT Wed Aug 18 2021/ 

NEAR TERM UPDATE /Now Through Thursday/...A typical summer time pattern is
in store throughout the near term as upper level ridging begins to 
build in over the region from the western Atlantic. At the same 
time, Fred continues to move northeast over the Appalachians as it 
slowly transitions into a non-tropical low and a broad, positively 
tilted trough over the central CONUS moves northeast into the Great 
Lakes Region. However, the these features will stay well outside of 
the local area as the aforementioned upper level ridging sets up 
over the Southeast.

Isolated to scattered showers and storms are expected to develop 
offshore and along the coast this morning before developing inland 
this afternoon. Essentially, convection will follow the typical 
diurnal pattern. PWAT's this afternoon are forecast to range from 
1.8 to 2.2 inches so convective coverage will be slightly higher 
than it was today. We opted to cap coverage at high end scattered 
this afternoon due to the building heights and somewhat limited 
coverage on CAMs. Decent instability will be present this afternoon 
with guidance indicating MLCAPE between 1000-2500 J/kg and DCAPE 
over 1000 J/kg. Model soundings also indicate the presence of dry 
air in the midlevels so a few strong storms cannot be ruled out.

By Thursday the upper level ridge will encompass much of the 
northern Gulf Coast, stretching all the way from Texas to the 
southeast coast of the US. Meanwhile, a surface high pressure begins 
tracking westward across the northern Gulf on Thursday evening, 
causing PWAT's to decrease to around 1.5-1.8 inches offshore. 
Moisture will remain abundant inland, however, due to southwesterly 
flow, leaving PWAT's around 1.8 to 2.1 inches. Scattered showers and 
storms can therefore be expected once again on Thursday with 
convection developing along the seabreeze. 

Highs today and Thursday afternoons will be hot as they reach into 
the low to mid 90s. These temperatures in combination with the 
increased moisture will cause heat indices to soar into the 102-107 
degree range. We will continue to monitor heat indices as these 
values are just below advisory criteria. Lows tonight are forecast 
to dip down into the lower to mid 70s inland with readings in the 
mid 70s to around 80 degrees along the immediate coast.

Swell approaching the surf zone has decreased early this morning and 
the HIGH rip current risk along local beaches is no longer in 
effect. 1-2 ft/5-6 s period swells, high tidal ranges, and outgoing 
tide in the afternoon will still support a MODERATE rip current risk 
today. Long period swells associated with tropical cyclone Grace 
moving westward from the Yucatan Peninsula into the southwestern 
Gulf of Mexico are forecast to arrive along the northern Gulf Coast 
by early Thursday evening. The rip current risk will become HIGH 
again along local beaches Thursday evening/night into Friday and 
Friday night. 14/21

SHORT TERM /Thursday night Through Friday night/...Summertime
continues on through Friday. The upper level trough that brought
slightly higher rainfall chances on Thursday will be slowly
lifting northeast out of the Mid-Ohio valley. In response to the
upper trough, the large ridge over the eastern seaboard will slide
south and west across the northern Gulf Coast. By Friday, the
ridge axis will likely be directly over head. Moisture across the
area will be rather high with PWATS around 2.1 to 2.2 inches
across the area. The best moisture will likely be over the
interior counties and decreasing towards the coast as you move
towards the center of the upper ridge. This will likely result in
scattered to potentially numerous storms across the area with 
rain chances highest along and north of highway 84 becoming more 
scattered as you move south towards the coast. Stronger storms may
be tougher to come by as deep moist profiles will keep MLCape 
values around 1000 J/KG and Dcapes around 500 J/KG. The bigger 
issue will be the potential for training storms and localized 
flooding. Steep surface lapse rates will support storm 
development, and with weak synoptic flow only around 10 knots 
throughout the layer storms will be likely to remain in the same 
location or possibly train over similar areas. Given PWATS in 
excess of 2 inches leading to warm tropical rain processes, rain 
rates could near 2 to 4 inch per hour rates in the stronger storms
leading to rate driven flooding. The main caveat will be the 
presence of the upper ridge providing some subsidence across the 
area. Storms will quickly subside Friday night as the upper ridge 
takes hold. 

Temperatures will be seasonable with highs in the low 90s and lows
in the mid 70s. Heat indices will range in the 104 to 107 range
given the higher low level moisture values. A HIGH risk of rip
currents will likely exist during this period as swell from 
Tropical Storm Grace makes its way northward through the Gulf. 
BB/03

EXTENDED TERM /Saturday Through Tuesday/...A broad upper ridge over 
the southern plains and deep south shifts northward and expands 
significantly across much of the southern CONUS through Monday as an 
upper trof over the interior eastern states shifts eastward to be 
oriented just off the east coast. A combination of a surface ridge 
over the north central Gulf coast states shifting westward followed 
by a surface trof becoming better defined over the Carolinas into 
the area will result in a primarily northwest flow over the forecast 
area through the period. Precipitable water values remain around 2.0 
inches through the period, but model soundings show progressively 
drier air flowing into the area through at least Monday roughly 
beneath 750-800 mb. Have stayed with chance pops each day for now, 
although it's possible these may be somewhat overdone. Highs in the 
lower to mid 90s on Saturday trend higher by Sunday to the mid to 
upper 90s. This could result in heat index values approaching heat 
advisory criteria (108), but this could be offset by somewhat lower 
surface dewpoint values. Will continue to monitor. /22

MARINE...Light to occasionally moderate southerly flow will 
generally persist through the period, with the exception of light 
west to northwest flow during the late night and early morning hours 
late this week and this weekend. Tropical Storm Grace is forecast to 
strengthen to a hurricane and track westward from the Yucatan 
Peninsula into the southwestern Gulf of Mexico Thursday night into 
Friday. Long period swells associated with this system will bring 
building seas late this week. /21

&&

.MOB WATCHES/WARNINGS/ADVISORIES...
AL...None.
FL...None.
MS...None.
GM...None.
&&

$$

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