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Area Forecast Discussion (AFD)

Issued by Huntsville, AL (HUN)

FXUS64 KHUN 190336

Area Forecast Discussion
National Weather Service Huntsville AL
1036 PM CDT Sat May 18 2019

.NEAR TERM...(Rest of tonight)
Issued at 1035 PM CDT Sat May 18 2019

Regional radar indicates that an outflow dominant QLCS continues to
push further east into western TN and north central MS. This is
farther east in expectation than recent HRRR runs and the 18Z 
NAM/GFS. The line should encounter a much drier and capped airmass as
noted in our earlier evening update. But can't ignore the current 
radar and have opted to add a low PoP of showers tonight in our far 
western portion of the forecast area (NW AL/Shoals area). 

.SHORT TERM...(Sunday through Monday night)
Issued at 310 PM CDT Sat May 18 2019

The overall wx pattern is xpcted to turn a little more active going 
into the second half of the weekend period. An upper trough axis over
the mid Plains states is xpcted to pivot NE into the Great Lakes on 
Sun. As this occurs, a sfc low will also move thru the Great Lakes 
states, dragging a weak frontal boundary ewd into the Mid South/lower
MS Valley regions. As the sfc low continues to lift to the NE, the
front will also translate ewd late Sun into Sun night. Sfc flow ahead
of the front may be a little strong, as evidenced by an H85 jet out
of the SSW near 40-50kt early Sun. This may translate into gradient
wind gusts around 30 MPH Sun morning in the higher elevations of NE
AL. Low level convergent flow along the front itself may result in a
few strong tstms Sun afternoon into Sun night capable of brief wind 
gusts to 50 MPH. The front is then xpcted to stall invof the area, 
with a few showers/tstms remaining possible into the day Mon. Even
with the front settling into the area, overall temps look to trend
unseasonably warm, given a weak upper ridge pattern remaining in
place over much of the Gulf region. Afternoon highs Sun/Mon look to
remain in the lower/mid 80s while overnight lows trend in the
lower/mid 60s for most locations.

.LONG TERM...(Tuesday through Friday)
Issued at 310 PM CDT Sat May 18 2019

At the beginning of the long term portion of the forecast on Tuesday
morning, sfc high pressure will be situated to our north while
another strong low pressure system will be taking shape well to our
west over the central High Plains. As the sfc high to our north shifts
eastward, flow will begin to back from the east and then south during
the morning. A strengthening low-lvl temperature gradient will give
rise to warm frontogenesis, but this is likely to take place just to
our north. Although, there is still some uncertainty in the location
of the developing warm front. Nevertheless, an axis of generally weak
confluence will be present over the area during the morning into the
afternoon. Global models also indicate a weak upper shortwave 
translating SSW-NNE across the area along the western flank of a 
building upper ridge. Although broad scale forcing will be generally
low, some showers and thunderstorms may develop in the moderately
unstable environment. A slight chance POP was thus introduced into
the forecast for Tuesday afternoon. 

Otherwise, the long term portion of the forecast will be dominated by
a highly amplified pattern across the CONUS with a deep trough in the
West CONUS and a strong upper ridge situated over the East CONUS 
from Wednesday into next weekend. This will bring generally dry and 
hot weather to the TN Valley region during the period. Given the 
general agreement among the suite of global and ensemble guidance,
this is a fairly high confidence forecast for the period. As
stated in the previous long term discussion, GEFS probabilities
indicate high confidence that 500 mb heights will exceed 2 standard
deviations above the mean for this time of year across our area. What
does this mean? Well, good confidence in above normal temperatures.
While the guidance suite shows a range of temperatures generally in
the mid 80s to lower 90s. Given the anomalous nature of the
situation, the actual forecast high temperature values were placed 
above the main part of guidance, with values reaching into the mid 
90s at some locations for Thursday through Saturday. Based on some of
the data, temperatures even reaching the upper 90s would appear
possible. Record high temperatures during this period are mainly in
the mid 90s, so some records could be broken. While there is high
confidence in the broad scale strong upper ridge, a few small showers
or storms could occur each afternoon during the period. However,
given the general limitation for broad scale ascent, any convection
will largely be driven by very localized effects...potentially sfc 
temperature/moisture gradients. Anyway, the risk for showers and 
thunderstorms currently is too low to place POPs in the forecast, but
will mention here for now. 


.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 525 PM CDT Sat May 18 2019

VFR flight weather conditions are expected through tonight into
Sunday morning. Southerly flow will increase markedly by 14Z with
sustained speeds of 12-17kt and gusts of 22-25kt. An area of showers
and a few thunderstorms will arrive in northwest AL including KMSL as
early as 14Z, but more likely 17-21Z. It is much less certain if
these will reach the KHSV area due to a more stable and drier airmass
further east, but have maintained VCSH after 19Z.





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