Scientific Forecaster Discussion
Area forecast discussion
National Weather Service La Crosse WI
1030 PM CDT Thursday Aug 27 2015
Short term...(tonight through Friday night)
issued at 253 PM CDT Thursday Aug 27 2015
Main forecast concerns this period...shra/tsra chances and rain amounts
through the period.
Data analysis at 18z had low pressure over south-central Nebraska...in
response to a shortwave trough and pieces of energy moving east from
Wyoming into SD/neb. Stronger of the 925-700mb moisture transport/Theta-E
convergence ahead of this low/trough was focused into eastern
South Dakota/southwest Minnesota early this afternoon. Rain showers/thunderstorms and rain on the increase in
far southwest Minnesota as of 18z...while precipitation elsewhere across the
region was waning. More clouds across much of the upper Midwest
today ahead of the plains low and shortwave trough coming east...but
increasing southerly surface-850mb flow spreading warmer and more moist
air into the region.
No major problems noted with 27.12z model initializations. GFS was
around 5f too low with surface dew points in the moisture plume ahead of
the plains low. Per WV imagery...NAM/GFS/can-Gem appeared to strong
with a shortwave initialized over northeast Colorado. Even with some
smaller differences at the start...models generally converging on a
tighter consensus as the Wyoming/Colorado shortwave energy merges and develops
a small middle level low over southwest Minnesota by 12z Friday. Trend at 12z Friday
is a bit weaker with the middle level low. Trend Friday favors faster of
the earlier runs with this low then opening up and moving east
across the region Friday/Friday night. GFS with the better run-to-run
consistency later tonight into Friday night. Short-term forecast confidence
is generally good this cycle.
For the short term...bigger question later tonight into Friday night is
how much is it going to rain and where will the heaviest rain fall.
Lower level thermodynamic forcing/lift continues to come together
for later tonight into Friday night. This with stronger lift/divergence
aloft ahead of the shortwave/trough and between partially coupled
250-300mb jet maxes. This deep layered forcing/lift along...with precipitable water
values in the 1.5 to 1.75 inch range and MDT/strong 925-700mb
moisture transport/Theta-E convergence of this higher precipitable water airmass
into the area...spreads across the forecast area centered on Friday. Model
consensus remains good for this system to produce a band of 1 to 3
inches of rain across the forecast area. The trend toward a weaker
overall 500mb low/trough and the stronger Colorado/southern shortwave has
shifted the axis of heavier rain southward...now more over south 2/3
of the forecast area...along/south of I-90. The slightly weaker system
also delays arrival of the rain into the area a bit tonight.
Lowered/slowed rain chances spreading across the area this evening
then raised them toward 100 percent late tonight/Friday as the
strongest/deepest lift moves across the area. With a faster opening
up of the trough and exit east...trended rain chances down more
quickly Friday night. Model consensus continues to show little if any
MUCAPE over the area tonight/Fri. Any thunderstorms and rain tonight/Friday look to have
to be driven by the stronger/ deep forcing/lift. Appears much of the
precipitation will come across the area as a broad shield/band of rain/rain showers
with embedded/isolated thunderstorms and rain. Limited thunderstorms and rain mention tonight/ Friday/Friday
night to isolated. Used a blend of the guidance lows/highs for
tonight through Friday night. A cool day Friday with rain/thick clouds over
the area much of the day.
Long term...(saturday through thursday)
issued at 253 PM CDT Thursday Aug 27 2015
For Saturday through Sunday night...
Main forecast concerns this period...lingering rain showers chances Sat
morning...temperatures through the period.
27.12z model runs consistent in the middle level trough axis moving
east of the area Sat morning. Models remain in decent agreement
with the trough axis to remain over eastern WI Sat afternoon as a
secondary/northern stream shortwave would drop down the west side
of the trough. Model then in good agreement for rising heights and
stronger middle level ridging to build quickly into the upper Midwest
Sat night into Sun night. Trend favors a compromise of the early
runs on the strength/placement of the ridging aloft by sun/Sun
night. Forecast confidence for Sat through Sun night is good this cycle.
Faster opening/movement east of the 500mb trough has the deeper
forcing/lift well east of the area by late Sat morning. Limited
most of the lingering rain showers chances with the exiting system to the
12-15z period Sat morning. Did linger a small -shra chance over
the far east/southeast side of the forecast area Sat afternoon...which
blends well with neighboring grids. This due to the stronger
secondary/northern stream shortwave dropping south across WI Sat
afternoon...some lingering lower level moisture and rather deep
cyclonic flow. This exits quickly Sat evening...with high pressure
and the ridging aloft then building across the region through Sun
night. May yet need some late night/early morning valley fog
mention centered on 12z sun and Monday. Will leave this out for now
and let those potential details come into better focus once the
system for Friday is by the region. Weak low level cold advection
behind the low for Sat...with weak/neutral temperature advections
for Sat night through Sun night. 925mb temperatures in the 16c-20c range Sat
afternoon...supporting highs mainly in the 70s. With the rising
heights/warming aloft...lower level airmass looks to warm in place
sun...with 925mb temperatures Sun afternoon supporting highs in the upper
70s/lower 80s. Used a blend of the guidance highs/lows for Sat/Sat
night and sun. Trended toward warmer of guidance highs Sunday with
plenty of sunshine expected and the ridge building aloft.
For Monday through Thursday...days 4 ot 7...
Main forecast concerns this period...temperatures and how warm will it
get by the middle of next week.
Medium range model runs of 27.00z/27.12z in rather good agreement
for middle level ridging to build over the region Monday. Good agreement
for he upper Midwest/Great Lakes to then persist through the week as a
Rex-type blocky flow pattern develops over noam. With decent and
improving model consensus...day 4-7 forecast confidence is average to
good this cycle.
The ridging aloft/Rex-block pattern would look to set the forecast area
up for a warm/dry period next week. However...the area is close to
the perimeter of the ridging for Tue-Thu. Persistent troughing on
the West Coast and southwest flow aloft across The Rockies and
plains will send shortwave ripples through the flow and into the upper
Midwest. Cannot rule out a stronger one of these waves spreading a
chance of rain showers/thunderstorms and rain into mainly the north half of the forecast area from
Tuesday Onward. One of these waves could develop a convective complex
over ND/northwest Minnesota any of the nights...with corfidi vectors
indicating propagation to the southeast/south across Minnesota/WI during
the overnight/early morning hours. Will stay with the consensus and
leave Monday-Thursday mainly dry for now. Persistent south-southwest low
level flow/warm advection raises 925mb temperatures into the 24c-28c range
for the Monday-Tuesday period...1 to 2 Standard deviations above normal.
Monday-Thursday continues to trend toward a very warm period with temperatures of
10f to 15f above normal...especially the highs. Raised highs Monday-Thursday
a bit over the consensus blend...mainly in the river valleys.
Consensus blend of the day 4-7 guidance lows looks good for now.
Aviation...(for the 06z tafs through 06z Friday night)
issued at 1030 PM CDT Thursday Aug 27 2015
Rain looks assured for krst/klse as shortwave-strong frontogenetics
interact with a fetch of southern moisture to create areas of rain
from overnight through the better part of Friday. Instability is
weak to none...but forcing strong enough for some isolated thunder
possibilities. Not high enough of a threat to include in forecast
though. Expect prolonged period of LIFR/IFR/MVFR ceilings/visibilities
associated with the rain...with the low ceilings/visibilities holding into the
overnight hours Friday. Timing has been the main forecast
focus...with model trends continuing to slow the system down. Latest
NAM suggests this...matching reasonably well with the hopwrf/hrrr-
CR. Will make minor adjustments to timing.
Hydrology...tonight through Friday night
issued at 346 PM CDT Thursday Aug 27 2015
Model/ensemble consensus continues to produce 1 up to 3 inches of
rain across much of the forecast area from later tonight through Friday
night. This falling across a landscape where most of the crops/
vegetation are in need of a good drink of water. One hour flash
flood guidance values are in the 1.8 to 2.7 inch range while 6hr
flash flood guidance values are in the 2.8 to 4.5 inch range. Most of the
precipitation expected to fall as rain and showers with isolated thunderstorms and rain.
Most rain rates looking to be in the 1/10 to 1/3 inch/hour
range...perhaps up to 3/4 inch/hour with any thunderstorms and rain. Given the
generally dry antecedent conditions and mainly moderate rain
rates...most of the rainfall late tonight through Friday night should to
soak into the soil with minimal runoff expected.