Scientific Forecaster Discussion
fxus64 khun 230526
Area forecast discussion
National Weather Service Huntsville Alabama
1226 am CDT Wed Aug 23 2017
for 06z tafs.
issued at 920 PM CDT Tue Aug 22 2017
The line of storms arrived a bit quicker than forecast and have made
adjustments to pop/wx grids to reflect this. Radar as of 9 PM shows
an outflow boundary ahead of the main line of showers/storms and this
has allowed for a weakening trend in the storms. There was good
development all along the line a few hours ago but now the strongest
storms are confined to along the Alabama/MS border. Reports so far
indicate wind gusts upwards of 35 mph possible with the storms/gust
front as they continue to move east/southeast through the Tennessee Valley.
Not overly confident in the forecast position/movement of the front
overnight as latest hires guidance and model suites are having a
terrible time depicting the convection along the front and are
generally too slow with the movement. The parent low associated with
the front is north of Maine, so any driving force to keep it moving
east/southeast through the area is limited. Could see a situation where the
convection continues to weaken through the evening hours as it moves
across the Tennessee Valley and then late tonight ends up stalling over the
southern half of the area. Surface obs show the drier air is well off
to the northwest and may not make it into the area until during the day
Wednesday. So, the heavy rain with the storms could potentially lead
to some fog/low clouds across the Tennessee Valley Wednesday morning. Plan
to wait another few hours before adding that to the forecast. Want to
see where the front ends up before fog at this time.
Short term...(wednesday through thursday)
issued at 241 PM CDT Tue Aug 22 2017
The surface front will push well into the Tennessee Valley Wednesday
morning. The aforementioned low level jet will shift east of the region by
12z, allowing for a break in coverage during the morning hours.
However, the front will serve as a focus for additional scattered
showers and thunderstorms from the late morning through the mid to
late afternoon, especially along and south of the Tennessee River.
Precipitation chances will wane quickly by late afternoon into the
evening as a push of drier air moves into the area from the north.
This will allow for some quick clearing by 00z and good radiational
cooling overnight on Wednesday given the lower dewpoints and drier
air mass in place. Thus, lows by early Thursday morning will be
notably cooler and more comfortable -- with a few upper 50s readings
possible across southern middle Tennessee! Other locations will
bottom out in the lower 60s. The drier air mass will make for an
early taste of September weather, with rhs in the 40 to 50 percent
range and highs in the low to mid 80s!
Long term...(thursday night through monday)
issued at 241 PM CDT Tue Aug 22 2017
12z Friday Onward...
Well, what a difference 24 hours makes! Compared to this time
yesterday, several details have come into much better light, and
with regards to others, more uncertainty has emerged for next week.
Most of the conundrum surrounding the extended portion of the
forecast revolves around the remnants of ts Harvey, and it's
eventual track across the southeastern U.S. Therefore, expect
several adjustments to this portion of the forecast over the coming
days as enhancements are made to the track of the decaying tropical
Friday begins this period with little fanfare as the recently
departed fropa is well S of the cwfa, with lower dewpoints filtering
into the area from the north. Sfc high pressure skirting across the
Great Lakes will keep quiet weather and north/northwest winds in place for the
Tennessee Valley, as all eyes begin to turn to the wrn Gulf and ts Harvey
taking aim at Houston, Texas. A vort Max rotating around ts Harvey will
meander NE through the day Saturday, blossoming showers/storms
across our County warning forecast area by midday. This convection will linger into the
early evening hours before dissipating shortly after sunset. Sunday
will be a near repeat of Saturday, but with slightly greater
coverage of showers/storms across the area. This will be thanks to
additional moisture advection in place along with typical
diurnally/orographically induced convection.
By this point in the forecast, a second tropical system will begin
to spin up at the tail end of the frontal boundary to our southeast. This
system should take shape over NE Florida by Sunday evening, eventually
pushing east/NE off the East Coast through midweek. As this occurs, the
remnants of ts Harvey are forecast to wobble east/NE across MS and into
central Tennessee. This is typically a climatologically-favorable track for
the Tennessee Valley to experience severe weather from a landfalling
tropical system, but there are many uncertainties regarding its
exact strength/track by the early to middle part of next week to
pinpoint that this far out. The potential for flash flooding exists
as well, taking into account the higher than normal pws (upwards of
1.75-2", which is near the 90th percentile of precipitable water climo.) And the
fact that we've been wetter than normal for this time of year.
To end the extended portion of the forecast, what's left of Harvey
traverses the se, taking a track that puts the remnant-center just west
of the County warning forecast area. This will make for a fairly wet start to the new work
week, with rainfall totals from Saturday and beyond around 3" in northwest
Alabama and at least 1.5" in NE Alabama (again, all depending on the track of
the system; everything in the extended hinges completely on that).
Given the enhanced chances for showers/storms from midday Saturday
and beyond, temperatures are expected to be below normal, only
topping out in the upper 70s to lower 80s in the afternoons, after
starting out in the middle 60s each morning.
Aviation...(for the 06z tafs through 06z Wednesday night)
issued at 1031 PM CDT Tue Aug 22 2017
Storms have dissipated leaving just a lingering shower or two near
the terminals but they are not expected to have any impacts.
Lingering cloud cover from the storms could keep the development of
low clouds/fog at Bay until close to 08-09z. These will most likely
impact the mean sea level terminal as a result of the earlier heavy rainfall and
cigs/vis could drop to IFR. However, still uncertain due to poor
performance from guidance this evening. Have kept the MVFR cigs/vis
starting at 08z but this may need to be updated.
The front that generated the storms should be south of the terminals
during the day Wednesday with VFR cigs and northerly winds between