Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 281149

Area Forecast Discussion
National Weather Service Huntsville AL
649 AM CDT Fri Jul 28 2017

For 12Z TAFS and quick near term update.


.NEAR TERM...(Today)
Issued at 649 AM CDT Fri Jul 28 2017

The start time of the flash flood watch was pushed up to 6 AM this
morning due to the developing heavy rain/flash flood threat evolving
over southern Tennessee. A persistent band of very heavy rainfall has
developed from Decatur to Giles county, and Multi-Radar/Multi-Sensor
estimates suggest rainfall amounts approaching 5 inches in the last
hour alone in the western part of that band. The band has formed in a
classic flash flood setup, with a 30-knot 850mb jet feeding the rain
from the west, a clear convergence axis, and precipitable water
values around 2.2 inches per satellite.

This band is beginning to push into northern parts of the area, near
Lincoln, Limestone, and Madison counties, so it will bear monitoring.
The 10z HRRR has a fairly good handle on the situation (although it
is underdoing the amounts) and it pushes a broader though still heavy
band of rain into the northeast half of the area thru 14-15z.

.SHORT TERM...(Tonight)
Issued at 335 AM CDT Fri Jul 28 2017

The front will pass across the area rapidly, with the initial wind
shift occurring by 06Z and final wind shift by daybreak. Showers
will linger through about midnight but should taper off rapidly
afterwards as the deep moisture is quickly shunted to the south (in
fact PWats will fall to around 1 inch by 12Z Saturday). The forecast
will continue to show mid-range PoPs early this evening but taper
off to 10% by daybreak tomorrow. The cooler and drier low-level air
will be slower to arrive, so lows should only fall to the upper
60s/around 70.

.LONG TERM...(Saturday through Thursday)
Issued at 335 AM CDT Fri Jul 28 2017

The cold front will be well to the south of the forecast area by
sunrise on Saturday, with high pressure building southward through
the day. The northerly flow at the surface will filter in a much
drier airmass, with dewpoints dropping into the lower 60s by the
afternoon. This airmass will behind the front will also bring some
cooler temperatures, with highs barely reaching into the mid 80s.
Thus, a rather pleasant Saturday is in store for the TN Valley.
Clear skies will prevail through the overnight hours, and as winds
become calm, temperatures will plummet into the lower 60s, with a
few spots even dropping into the upper 50s early Sunday morning.

The dry conditions will prevail through much of the long term
period, as high pressure dominates at the surface. Upper level
ridging tries to build into the region on Sunday, however it will
have a hard time overcoming the broad upper trough that will be in
place over the eastern CONUS. This keeps the TN Valley within
northwesterly flow through at least Wednesday. However, winds don`t
really shift back to the south until late Tuesday, and flow will be
rather light, limiting the amount of moisture advection. By
Wednesday, a shortwave will dig southward and amplify across the
Ohio and Tennessee Valleys. Despite some potential forcing on
Tuesday and Wednesday, the lack of moisture will limit any
thunderstorms from developing. Thus, have kept a dry forecast in
through Wednesday. A return of diurnally driven thunderstorms will
begin on Thursday, as moisture continues to increase and finally
reaches above climatological averages for the beginning of August.
This may be aided by a weak frontal boundary that the ECMWF shows
moving through the forecast area. However, not overly confident on
the front making it through on day 7 and will only include isolated
thunderstorms at this point.

Temperatures through the first half of next week will remain slightly
below normal, with highs in the lower to mid 80s Sunday through
Tuesday. However a moderate trend, due to some return flow, will
send temperatures towards more normal values by Wednesday and highs
will warm to around 90 degrees on Wednesday and Thursday.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 649 AM CDT Fri Jul 28 2017

KHSV is likely to see poorer flight conditions early this morning as
it will be closer to a persistent band of showers/storms. Initially
this will be handled with MVFR showers and TEMPO group for heavier
thunder, with improving conditions at 15z as the storms become more
scattered. Confidence in improvement timing is low to medium.

For KMSL, scattered thunderstorms will still affect the terminal at
first, but it will not be quite as persistent or heavy, and
conditions will improve with scattered thunderstorms by 14z.

After 14-15z, lower clouds around 1000ft could develop, although
confidence is not high enough to include beyond a scattered deck at
either site. The frontal passage will occur 00-01z with a return to
VFR at 08-09z.


AL...Flash Flood Watch through this evening for ALZ001>010-016.

TN...Flash Flood Watch through this evening for TNZ076-096-097.




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