Area Forecast Discussion
Issued by NWS Mobile, AL

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FXUS64 KMOB 151704 AAA

Area Forecast Discussion...UPDATED
National Weather Service Mobile AL
1104 AM CST Fri Dec 15 2017

.DISCUSSION...See updated information for land areas below.


.UPDATE...Upper air maps show trof axis aligned from the Great
Lakes, southward over the Mississippi Valley, then southwest
across the Red River Valley of Texas/Oklahoma. East of this
feature, east Pacific moisture is tapped with large area of
clouds streaming northeast over the deep south. Surface cold front
has settled into the central Gulf with high pressure over Texas
beginning to expand eastward into the deep south. With the
expansive cloud cover to hold in place in the presence of lower
1000-850 mb thickness values and latest trends in observations,
have made some minor, lower adjustments to daily highs with
numbers possibly struggling to get out of the 40s over the
northern tier of zones. Otherwise, lower to mid 50s. These values
are well below seasonal for the middle of December. Radar shows
some light returns spreading east northeast over the local area.
Will add a very small pop in for this, and will mention light
drizzle. Measurable light rain seems unlikely as soundings along
the central Gulf coast show larger temperature/dewpoint spreads in
the lowest 10 kft, which will aid in evaporation of precipitation
falling thru this column. Of note, evaporative cooling in this
layer also supports keeping daytime highs on the cool/chilly side
as well. /10


.PREV DISCUSSION... /issued 528 AM CST Fri Dec 15 2017/

DISCUSSION...Updated for latest aviation discussion below.

12Z issuance...VFR CIGs/VISBYs expected through the forecast. Shra
will pass east along and south of the coast, but be of little to
no consequence at area TAF sites.


PREV DISCUSSION... /issued 454 AM CST Fri Dec 15 2017/

NEAR TERM /Now Through Friday night/...Shortwave energy streaming
e-ne south of a more organized shortwave trough easing slowly
south over the Southeast. As the day progresses, the slowly
southward moving energy becomes dominant. This energy works with a
front stalled just south of the marine portion of the forecast
area to provide the focus for isentropic upglide shra to mainly
the marine portion of the area today into this evening. Am
expecting best chance of rain reaching the surface being along and
south of the coast, decreasing in coverage and intensity as the
southward moving energy becomes dominant. Looking down-stream,
over the Lower Mississippi River Valley, the current "precip"
showing on radar looks to be mainly mid to upper level based
virga, with ceilings reported at or above 12,000` except for obs
south of New Orleans, re-enforcing the mainly marine rain today.
With abundant cloud over the area and cold advection expected
through the near term, temps well below seasonal averages likely,
with a light freeze possible over the northwestern two-third of
the land portion of the forecast area. /16

SHORT TERM /Saturday Through Sunday night/...A 1026mb surface
high pressure area across the southeast states Saturday morning
will move east, setting up a southerly return flow over the
weekend. The region will remain wedged between upper high pressure
centered over the Bahamas and an upper cutoff low over northwest
Mexico lifting northeast over west Texas through midnight
Saturday. The upper low will then move quickly toward the
northeast conus as an open wave late Saturday night and Sunday,
disippating along the northern periphery of the upper high
pressure area by Sunday evening. Deep layer moisture will increase
significantly across the region, with precipitable water values
climbing to between 1.8 to 2 inches by Sunday evening.

Rain-free conditions will persist across the forecast area
through Saturday night, followed by numerous to definite rain
chances, along with embedded thunderstorms, spreading from west
to east across the forecast area Sunday and Sunday night as upper
level impulses associated with the ejecting upper wave traverse
the region. Widespread rainfall amounts on Sunday will range from
0.75 to 1.25 inches, with locally high amounts near 2 inches

A rapid rise in temperatures will also occur as low temperatures
Saturday morning in the upper 20 to mid 30s climb into the upper
50s to mid 60s for lows Sunday night. High temperatures in the mid
to upper 50s on Saturday will climb into the mid 60s to lower 70s
on Sunday. /22

LONG TERM /Monday Through Thursday/...The pattern then gets
increasingly complicated through the long term as a second cutoff
upper low pressure area over the southwest conus Monday morning
moves slowly eastward and also evolves into an open wave over the
southern plains on Tuesday. A more southern track of the upper
level impulses east of the open wave are anticipated to move over
the southeast states due to the upper high pressure area migrating
toward the southwest.

The ECMWF develops two upper shortwaves that pass just north of
the forecast area, while the GFS maintains one stronger upper
shortwave over the deep south. Timing differences of upper level
impulses between the two models create low confidence on exactly
when the higher chances of precipitation will occur. For now,
forecasting numerous to definite rain chances along with embedded
thunderstorms on Monday, followed by scattered to numerous rain
showers Monday night through midweek, with decreasing
thunderstorm coverage. Lower rain chances should occur Thursday as
most of the energy will have moved east of the region.

Additional widespread rainfall amounts Monday through Thursday
are expected to range from 1.00 to 2.00 inches, with locally high
amounts near 3 inches possible. /22




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