Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 301144

Area Forecast Discussion
National Weather Service Huntsville AL
644 AM CDT Tue May 30 2017

For 12Z TAFS.


.NEAR TERM...(Today)
Issued at 512 AM CDT Tue May 30 2017

West-southwesterly mid-level flow continues this morning, as the
local forecast area remains positioned between an upper low located
across Ontario and a subtropical ridge centered northeast of the
Bahamas. Both latest water vapor imagery and near term model guidance
suggest the presence of a weak vorticity maxima embedded within this
flow...with this feature currently crossing the central Gulf coast. A
broad region of rain and embedded convection associated with the
disturbance to our south has been confined to central/south AL thus
far this morning...with a separate axis of scattered thunderstorms
related to a southeastward drifting cold front extending from the
Ozarks into southern KY-TN. With little threat from either of these
areas, we expect dry conditions to prevail through sunrise. Patchy
fog will continue to develop in the moist airmass, especially as
cirrostratus clouds spread eastward and skies become clear.

Models are in agreement that the previously mentioned mid-level wave
impacting the central Gulf coast will shear apart and lift
northeastward across GA/SC later today. However, the northern edge of
the precipitation shield associated with this disturbance may clip
our southeastern counties this morning with some light rain and
thunderstorms. Meanwhile the cold front to our north is expected to
loose definition as it drifts southward into northern TN. Additional
convection will likely begin to develop early this afternoon in the
warm/moist airmass south of the cold front, but with weak synoptic
scale subsidence expected in the wake of the disturbance to our south
and little to no evidence of low-level convergence there is nothing
to warrant a POP greater than 30%. Any deeper updrafts that manage
to develop should form in an environment with 1500-2500 J/kg MUCAPE
and 25-30 knots of bulk shear...which may yield loosely organized
multicell clusters producing brief strong wind gusts. With less
cloud cover and precipitation, temperatures should be several degrees
warmer today...with highs in the l/m 80s.

.SHORT TERM...(Tonight through Thursday night)
Issued at 512 AM CDT Tue May 30 2017

Throughout the remainder of the short term period, the subtropical
ridge is forecast to gradually retrograde across the Gulf of
Mexico...while the upper low across Ontario becomes absorbed in the
circulation associated with a stronger trough dropping southward
along the western coast of Hudson Bay. This will result in gradual
veering of flow aloft to the west tonight and to the west-northwest
on Wednesday. At the surface, an anticyclone predicted to build
southward into the northern Plains today will continue to push
southeastward into the lower OH valley on Wednesday...effectively
pushing the diffuse cold front through our region at some point
during the late morning/early afternoon. Prior to the arrival of the
front, we anticipate another warm/muggy night tonight...with
scattered thunderstorms dissipating within a few hours of sunset.
Conditions appear favorable for the development of patchy fog once
again as temperatures fall into the l/m 60s...and this has been
included in the grids. The coverage of thunderstorms will likely be a
bit higher on Wednesday based on expectations of frontal passage
during this period, but height rises aloft progged by all guidance
should keep convection scattered at best. With temperatures expected
to once again reach the m80s and forecast soundings indicating
similar values of CAPE and shear, a few strong storms will be
possible on Wednesday as well.

A brief intrusion of drier air will occur on Wednesday night and
Thursday morning as the surface high settles into the central and
southern Appalachians. Resulting lows are expected to be the coolest
of the forecast period, with temps around sunrise expected to range
from the u50s in the northeast to the l60s elsewhere. Although patchy
fog is certainly a possibility, we have not included it in the
forecast at this point due to uncertainties regarding the degree of
dry air advection. Flow aloft will begin to back to the west on
Thursday, as a weak 500-mb shortwave trough lift northeastward across
the southern Plains. The southern end of this sheared trough is
expected to push northeastward into the lower MS valley late in the
day...perhaps resulting in an increase in convection across our
southwestern zones by late afternoon. However, the greatest lift and
moisture advection will begin Thursday evening...with scattered
showers and thunderstorms expected overnight as PWAT values rise back
to around 1.6 inches. Lows on Thursday night will respond to the
increase in low-level moisture and cloud cover by only dropping into
the m60s for most locations.

.LONG TERM...(Friday through Monday)
Issued at 512 AM CDT Tue May 30 2017

High pressure at the surface and aloft should control the general
weather situation for the later portion of the week. A persistent
southerly flow will continue a feed of moisture from the Gulf of
Mexico across the southeastern states, keeping some chance of
convection in the forecast into the upcoming weekend.

Atmosphere precipitable water amounts on Friday should be in the 1.5
inch range, but will increase to the 1.8 to nearly 2 inch range
during Saturday and Sunday. This, moisture convergence, and lift will
help bring very good rain chances, with locally heavy rainfall a
possibility during the weekend. This could result in hydrological
issues by early next week - stream and river level rises from
rainfall runoff. This far out, won`t really jump on this potential
yet, but want to bring up this potential. Given models were
advertising high rain chances, joined the bandwagon with likely rain
chances for Saturday and Sat night. Given more clouds than from the
latter portion of this week to early next week, kept daily high
temperatures a little below seasonable norms - in the lower rather
than seasonable mid 80s. Lows in the upper 60s Friday night/Saturday
night should cool into the mid 60s during Sunday night.

Overall storm strength appears will stay on the "general" side, with
the usual gusty winds, frequent lightning, and locally heavy
rainfall. CAPE values on Sunday should rise to near 1000 J/kg, but
overall shear values stay on the lower side. Timing wise from the
models, the Canadian was the fastest model bringing the front
southward, followed by the ECMWF, then the GFS. In any case, the rain
chances should taper off from north to south following the passage
of the cold front.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 644 AM CDT Tue May 30 2017

IR satellite data continues to indicate rapid clearing of a
cirrostratus cloud shield from w-to-e...allowing fog to impact both
terminals this morning. Based on current obs, a tempo for reduced
vsby has been included thru 30/13Z, with fog expected to dissipate
rather quickly by 8 am CDT. Otherwise, VFR conditions are anticipated
for remainder of the day...with high-based cu btwn 4-8 kft
developing by 16Z. Isolated-widely scattered shra/tsra will be psbl
this aftn and early evening...but due to a much lower coverage
compared to yesterday we will not include in the forecast at this
point. Afternoon clouds should dissipate within an hour or so of
sunset... with conds favorable for patchy fog development once again
late in the TAF period.





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