Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Memphis Tennessee
341 am CDT Wednesday Jul 29 2015
One more day of dangerous heat and humidity is expected on
Wednesday with relief coming in the form of a cold front moving
through the middle-south late Wednesday night/Thursday morning...
Currently...a very warm night was ongoing across the middle-south
with temperatures near 80 degrees and dewpoints in the middle to
upper 70s. Aloft...an upper level ridge was anchored across much
of the southern US. At the surface...high pressure was in place
across much of the southeastern US with a cold front oriented SW
to NE from the Texas Panhandle through northern MO/southern WI. Earlier in
the evening...a substantial complex of thunderstorms developed
across portions of middle Tennessee which expelled an outflow
boundary southwest into portions of the Tennessee River valley.
While surface winds are variable...that boundary is likely still
present through the region but becoming more diffuse with time.
For Wednesday...excessive heat is still forecast...however there
are a few complicating factors. Hi-res members including the NSSL
WRF and the hrrr develop thunderstorms early in the day as a weak
disturbance dives southeast around the periphery of the surface
ridge. The aforementioned outflow boundary may serve as a focus
for this development but this solution is extremely uncertain at
this point. If it were to unfold as such...portions of west
Tennessee along the Tennessee River may not reach heat advisory criteria
owing to cloud cover/precip. At this point...did not adjust the
advisory given the aforementioned uncertainty. Believe the hi-res
solutions to be much too aggressive with precipitation development...but
it is Worth mentioning.
Further west toward the MS river...excessive heat warnings remain
in effect across east and NE Arkansas/northwest MS/west Tennessee with high confidence in
dangerous heat indices developing from midday through early
evening. Many locations will experience several hours of heat
indices between 111 and 114 degrees given the extremely moist
boundary layer in place with dewpoints in the middle and upper 70s
and temperatures in the upper 90s. Precipitation is not expected
to hinder solar heating and should hold off until the evening
Wednesday night...the frontal boundary approaches from the north
but considerable uncertainty exists with respect to coverage of
showers and thunderstorms. There will likely be isolated
development after 00z Thursday along the front but placing this
development is difficult at this time. For now...slight chance to
chance probability of precipitation were maintained to account for this.
By Thursday...the frontal boundary will continue to push south
with drier air advecting into the region. Northern Mississippi may
still near heat advisory criteria earlier in the day but even this
should be short-lived as cloud cover and showers/thunderstorms should
provide relief during the middle to late afternoon hours. By Thursday
night...the front will have cleared the region leaving a much more
pleasant airmass in place with dewpoints in the low to middle 60s and
temperatures in the upper 60s to near 70 Thursday night.
Through the weekend...a slow warmup is expected but several days
of bearable temperatures are forecast with lower humidity. This
period should also be dry for the entire region.
In the long-term...upper level ridging strengthens across the
Desert Southwest with a persistent area of upper level low
pressure across the east-central Canadian provinces. The middle-south
will fall somewhere between these two features with warming
temperatures once again. A slow increase in heights suggests that
another warm to hot period may develop but it is too soon to tell
how hot...but it is Worth watching.
06z taf cycle
Patchy fog may develop late tonight producing temporary MVFR/IFR
conditions at mkl and MVFR conditions at tup. Otherwise...VFR
conditions expected to prevail at sites for most of the period.
Thunderstorms in the vicinity possible at sites after 29/12z and at least through 30/02z as
an upper level ridge begins to weaken slightly and a weak cold
front enters the lower Mississippi Valley.
Arkansas...excessive heat warning until 8 PM CDT this evening for Clay-
MO...excessive heat warning until 8 PM CDT this evening for Dunklin-
MS...excessive heat warning until 8 PM CDT this evening for Coahoma-
Heat advisory until 8 PM CDT this evening for Alcorn-Benton MS-
Tennessee...excessive heat warning until 8 PM CDT this evening for Dyer-
Heat advisory until 8 PM CDT this evening for Benton Tennessee-Carroll-