Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 230445
Area Forecast Discussion
National Weather Service La Crosse WI
1145 PM CDT FRI JUL 22 2016
.SHORT TERM...(Tonight thru Saturday night)
Issued at 305 PM CDT Fri Jul 22 2016
Main fcst concerns this period: heat/humidity thru this evening,
SHRA/TSRA chances late tonight thru Sat night, severe TSRA risk
centered on Sat evening.
18z data analysis had a weak frontal boundary from northeast WI to
near KLSE to northwest IA. Bubble of high pressure with some drier
low level air sinking south into the area behind the front. Moist/
unstable airmass remained in place south of the boundary. Weak
moisture transport/convergence south of the boundary resulting deck
of cu/strato-cu clouds over the central/south parts of the fcst
area. These clouds holding temps down many areas today, with early
afternoon heat indices mostly in the mid 90 to around 100 range.
No issues noted with 22.12z model initializations. Solutions
reasonably similar in lifting the shortwave energy over WY at mid-
day to northwest MN by 12z Sat as a strong shortwave trough crosses
the northern Rockies. Models favoring faster/bit further north of
their earlier runs on lifting this trough to near Lk Winnipeg by 00z
Sun then closing off a mid level low over central Manitoba by 12z
Sun. Given the good model agreement, short-term fcst confidence
remains on the good side this cycle.
In the short term, persistent 1500-2500 ft cloud deck in an area of
weak 925-850mb moisture transport/theta-e convergence, as seen in
the KMPX/KARX/KDMX 925-850mb VAD wind fields, continued to raise
havoc with excessive heat warning for much of the fcst area. Heat
indices across much of the area at mid afternoon mostly in the 95-
100 range, a bit short of the warning criteria but close to the
advisory criteria. Will just ride out the headline at this point.
Heat indices Sat afternoon may again flirt with the 100 range some of
the warmer MS/WI river valley locations and across the south end of
the fcst area. A heat advisory might be needed for portions of the
area Sat afternoon, but will not issue any headlines for Sat at this
time to avoid confusion with those in effect into this evening.
The weak moisture transport/theta-e convergence continues into the
southwest end of the fcst area late this afternoon/early this
evening. This as the weak sfc trough/frontal boundary continues to
sag south across the area thru this evening. Will continue with a
small SHRA/TSRA chance across roughly the southwest 1/3 of the fcst
area as cap is at its weakest and soundings showing lifting elevated
parcels with at little as 10 J/KG CIN. 0-6km shear on the order of 15-
20kts so main threat from late afternoon/early evening storms would
be heavy rain. Brief respite from the SHRA/TSRA chances late this
evening into early overnight hours. Southerly low level flow
increases later tonight, into/over the old frontal boundary which
starts to return north as a warm front. Plenty of sfc-850mb moisture
and CAPE in the warm sector south of this warm frontal boundary (PWs
in the 1.5 to 2 inch range) to be lifted over the boundary. Stronger
of the lower level thermo-dynamic forcing looks to remain west of
the fcst area thru tonight, then is progged to translate northeast
across the fcst area with the warm front Sat/Sat evening. Deep
layered shear remains on the weak side over the area much of Sat,
with main threat from any Sat storms again the heavy rain. Cold
front approaches later Sat evening with an increase of lower level
convergence/linear forcing along it as it pushes into the very
moist/unstable airmass. Deep layer shear increases as well for Sat
night. Depending on timing and convection along the cold front
organizing into line segments with strong cold pools, some of the
storms expected to move thru with the cold front could be severe
with damaging winds. Earlier arrival of the storms (late evening vs.
more well into the overnight hours) would pose a greater risk of
severe weather. For now, various large scale and hi-res models
providing a variety of timing and TSRA evolution outcomes Sat
afternoon/night. Will stay the course with consensus SHRA/TSRA
chances Sat/Sat night. Used a blend of the guidance lows/highs
for tonight thru Sat night.
.LONG TERM...(Sunday through Friday)
Issued at 305 PM CDT Fri Jul 22 2016
For Sunday thru Monday night: main fcst concerns this period include
lingering SHRA/TSRA chances Sun morning, arrival of a more
comfortable summer-time airmass.
22.12z model suite remains on track to move the mid level trough
axis across the region Sun, shortwave ridging/rising hgts to quickly
build across MN/IA/WI Sun night/Mon. Good agreement for hgts to then
fall again Mon night as shortwave energy coming across the northern
plains drops into broad troughing over eastern NOAM. Trend favors
faster of the earlier runs on the passage of the trough Sun into Sun
evening, with stronger west-northwest flow aloft over the region
much of this period. Fcst confidence in the Sun thru Mon night
period is good this cycle.
Models in good agreement for the sfc-850mb trough/front to be
pushing east of the fcst area around 12z Sun. If faster of models
are more correct with this passage, may be able remove all SHRA/TSRA
chances from Sun. For now lingered some small SHRA/TSRA chances
across the east end of the fcst area 12z-15z Sun. Deep layered
subsidence, sfc-850mb drying and low level cold advection in the
wake of the trough/front to spread across the area Sun/Sun night,
with the low level cold advection/drying continuing into Mon. After
a few days in the heat and humidity, appears some relief is in store
Sun/Mon as a modified Pacific high builds into/across the region.
Modest west/northwest winds to keep the lower levels stirred Sun/Sun
nigh but by Mon night the high is progged to be centered over the
area. After any lingering SHRA/TSRA chances Sun morning, this period
shaping up to be rather dry/quiet with temperatures trending back
toward normal by Mon/Mon night. May be some concerns for late
night/early morning valley fog around 12z Tue but plenty of time to
sort those details out. stayed near a blend of the guidance highs/
lows for Sun thru Mon night.
For Tuesday thru Friday (days 4 to 7): main fcst concerns this
period are SHRA/TSRA chances mainly Wed thru Fri.
Medium range model runs of 22.00z/22.12z is decent agreement for
quasi-zonal flow across the northern CONUS Tue/Wed. Lesser agreement
on the shortwave details in this flow, but some consensus for hgts
to fall across the Upper Midwest Tue/Wed as a mid level low develops
over the Hudson`s/Jame`s Bay area. Loose agreement for broad
troughing to develop over the eastern CONUS Thu/Fri as hgts rise
along the west coast and the mid level low drifts across eastern
Can. Plenty of detail differences on stronger shortwaves thru the
developing northwest flow aloft over the region Thu/Fri, but this
more-or-less the norm in northwest flow at a day 6/7 timeframe. Fcst
confidence is near/above average Tue/Wed then below average Thu/Fri.
Falling hgts/approaching pieces of shortwave energy Tue nudge the
quiet/comfortable high from Mon east of the region. Bulk of the
moisture return/moisture transport progged to stay west/north of the
fcst area until later Tue night/Wed. Overall lower level thermo-
dynamic forcing appears weak at the passing sfc-500mb trough is not
well defined at this time. SHRA/TSRA chances increasing into mostly
the 30-40% range by Wed/Wed night okay for now. Evolution/details of
passing shortwave in northwest flow much more in question Thu/Fri.
ECMWF brings the next one by on Thu, GFS brings the next one into
the region Fri. Both look to have enough moisture/forcing lift to
spread a respectable chance of SHRA/TSRA across the area as they
would approach/move thru. 20-40% SHRA/TSRA chances for much of the
area Thu/Fri reasonable until the timing details sort themselves out.
The increasing northwest flow thru next week, rounds of lower level
cool advection and potential for more cloud cover much of Wed thru
Fri trends temperatures downward thru the Tue-Fri period. By Thu/Fri
925-850mb temps looking to support highs in the upper 70s/low 80s.
Model/ensemble consensus of highs/lows with a cooling trend thru
next week look good at this time.
.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1145 PM CDT Fri Jul 22 2016
The biggest concern through sunrise remains the potential for some
fog development. Already seeing some spots dipping below 3SM for
visibility, and with low temp/dew point spreads currently at RST,
suspect they will again see a period of fog, though not sure it
will end up as dense as yesterday morning. LSE may also see some
fog, and a few periods of lower stratus are also possible just
Continue to believe there will be at least scattered showers and
storms developing through late morning/early afternoon along an
elevated warm front, but confidence in coverage remains
sufficiently low to only include a vicinity mention just yet.
Confidence is higher in a second line of storms arriving into the
evening hours (mainly after 03Z), and that feature may have a risk
for very strong winds as it passes.