Area forecast discussion
National Weather Service La Crosse WI
341 am CDT Monday may 20 2013
Short term...(today and tonight)
issued at 341 am CDT Monday may 20 2013
Main forecast concern is on severe weather and flooding potential
today.
Currently as of 08z...water vapor loop and rap 500mb height analysis
showed an upper low over South Dakota and Nebraska with ridging
ahead of it over Michigan. The upper low was mostly cut off with
ridging to its north over south central Canada. 500mb Standard
deviations were 1-1.5 below normal with the upper low. Numerous
shortwaves were ejecting in southwest flow ahead of the upper
low...some of which are convectively produced. One for example was
lifting through northwest Wisconsin which brought rain to much of
the area during the nighttime hours. Rap data also showed a low
level moisture transport zone with this mesoscale convective vortex. Farther south...another
low level moisture transport zone was noted in southern Missouri...
pointing into southern Illinois. Combination of subsidence in the
wake of the mesoscale convective vortex and split in moisture transport has allowed the
precipitation to diminish south of Highway 29. Plentiful moisture coming
up ahead of the upper low with precipitable water values of 1.2-1.5
inches from eastern OK into soutehrn WI. There was a minima of
precipitable water from western Kansas into much of Iowa...though....
associated with a dry slot south of the upper low. At the
surface...low pressure was situated under the upper low with a warm
front extending east to Eau Claire WI. Rap 850mb temperatures were in the
14-18c range south of the front in the forecast area.
Models are in good agreement of keeping the upper low nearly
stationary over South Dakota. Despite its stationary movement...the
weather will be quite a bit different compared to yesterday.
Primarily this is due to the upper flow ahead of the upper low
prognosticated more southwesterly versus southerly. Additionally...the
plume of higher precipitable water is prognosticated to slowly shift east
thanks to the upper low.
Most of the forecast area is indicated to stay in the warm sector
through tonight...along with existing in that southwest flow for
shortwaves to come through...both favorable for thunderstorm
development. However...one question mark is moisture. Based on a
amdar sounding out of Omaha at 04z...the 850mb dewpoints from the
20.00z NAM were 6-7c too high...thus the NAM builds a lot of cape
quickly this morning. The rap is much more subdued having the area
encompassed by the dry air...especially when looking at MLCAPE.
Thinking the rap idea is much more representative and have kept the
area dry through early afternoon. Eventually...the rap does catch up
with the cape late in the afternoon thanks to evapotransporation of
moist soils. In fact...the rap shows a definitive dry line in the
warm sector forming near I-35 in the afternoon. This dry line should
serve as a trigger for convective development...especially in
southeast Minnesota and northeast Iowa. Majority of models...including hi
resolution ones...show this scenario...with the thunderstorms then
propagating northeast through the evening. The best 0-6km shear of
greater than 35 kts is prognosticated over the southeast half of the
forecast area...opposite of where most of the convection will be. On
the other hand...freezing level heights are lower on the order of
10500 feet where the convection is expected...thus there could end up
being marginally severe hail. Storms should gradually weaken as
instability wanes late in the evening into the overnight hours. It
should be noted...if for some reason a storm fires in the higher
0-6km shear area this afternoon...it would likely become
supercellular. Again...though...think this is a low probability with
that area perhaps even being capped. See the hydrology section below
for flooding concerns.
Certaintly the additional rain from storms this afternoon and early
evening will not help the flooding matters in portions of southeast
Minnesota and northeast Iowa. Have kept the Flash Flood Watch going
to 00z this evening...which works out well in terms of time because
the convection should mostly be east of there by the expiration.
Should see quite a bit of sun today and with 850mb temperatures hovering in
that 14-18c range...temperatures should easily climb into the upper
70s to middle 80s. Another mild night likely tonight with plenty of
clouds and dewpoints remaining up in the 50s to low 60s.
Long term...(tuesday through sunday)
issued at 341 am CDT Monday may 20 2013
Tuesday through Thursday...
main focus here is on the evolution of the upper low. 20.00z
European model (ecmwf)/GFS/Canadian/NAM are all in pretty good agreement holding the
upper low nearly stationary on Tuesday...then gradually shifting
eastward Tuesday night and Wednesday before it gets more of a kick
east Wednesday night. The kick east is in response to a potent
trough dropping south through much of eastern Canada.
For Tuesday...much of the forecast area west of the Mississippi
River looks to be dry-slotted...thus have lowered precipitation
chances. To the east...there are indications that some diurnal
convection could form along a cold front marching east. Instability
is pretty meager in this case compared to what we have been dealing
with...thus any convection should not go severe. Better shot for
severe weather will be well south and east of the forecast area.
Highest precipitation chances overall are expected north of Highway 29
in a low to middle level frontogenesis zone.
As the upper low begins to push east towards the forecast area
Tuesday night and Wednesday...DPVA increases while simultaneously
the low to middle level frontogenesis zone drops south. Therefore...
increasing precipitation chances seem reasonable... especially on
Wednesday when the two forcing mechanisms are juxtaposted.
Additionally...there could be some diurnal enhancement on Wednesday
being close to or under the cold core of the upper low.
Models appear to be trending faster in kicking out the upper low for
Wednesday night and Thursday...enough so that Thursday may now end
up dry. Wednesday night still looks favorable for precipitation...
especially east of the Mississippi River which is in that
aforementioned juxtaposition of forcing mechanisms.
Definitely a cooling trend will take place for Tuesday through
Thursday with that upper low coming in...and then come Thursday a
northerly flow advecting air from Canadian high pressure trying to
build south into the upper Great Lakes. 850mb temperatures fall from 11-13c
at 12z Tuesday to 4-6c by 12z Thursday. Went with a blend of guidance for
highs and lows for now given no signal to lean towards warmer or
colder scenario given the forecast pattern.
Thursday night into next weekend...
20.00z European model (ecmwf)/GFS/Canadian and GFS ensembles continue to advertise a
blocked up upper level flow pattern across North America during the
extended portion of the forecast. The flow features deep troughing
along the West Coast...ridging from the plains into the northwest
territories...and mean troughing from eastern Canada into the
middle-Atlantic region. What this pattern means for the forecast area
is a battle between dry...Canadian high pressure and a more humid
airmass advecting north out of the Gulf of Mexico under the upper
ridge. It appears dry conditions should prevail for Thursday night
and Friday...resulting from subsidence in the wake of the early to
middle week upper low. From Friday night and beyond...have followed a
consensus approach and spread precipitation chances across the
forecast area...highest west of the Mississippi River. These chances
are a result of surges of warm advection. The precipitation could
become enhanced too at times as jet streaks propagate between the
eastern Canada trough and plains ridging.
Something to watch during the extended is potential for frost...
especially in central Wisconsin. Thursday night according to 20.00z
models would be the highest chance...though confidence is not high
enough to either mention frost in the forecast or drop lows below
40. Otherwise...temperatures should stay mostly below normal being
under the influence of the Canadian high and a northeast flow.
&&
Aviation...(for the 06z tafs through 06z Monday night)
issued at 115 am CDT Monday may 20 2013
It appears the last surge of rain and thunderstorms are moving
north through the taf sites at this time and for the next few
hours. Moderate rain and MVFR ceilings/visibilities will clear as the rain
moves off to the north. A clearing trend is expected with VFR
expected for the daylight hours.
Instability will again build over the taf sites and set the stage
for late afternoon and evening thunderstorms and rain chances. For now have included
a thunderstorms in the vicinity and future forecasts can refine the timing at the taf
sites. Monday afternoon and evening thunderstorms and rain coverage is not expected
to be as widespread or affect the taf sites as long as the
activity Sunday night.
&&
Hydrology...
issued at 341 am CDT Monday may 20 2013
Multiple days of heavy rain across portions of southeast Minnesota
and northeast Iowa has really lowered the threshold for flooding. A
stripe from Mitchell County Iowa through Mower...western Fillmore and
Olmsted counties were especially hard hit from yesterday\s rain.
Report from Mitchell County Iowa emergency manager at 330 am was that
most main roads are closed due to high water and many side roads are
washed out. Thus...the area that was put in a Flash Flood Watch
yesterday cannot handle any more rain. See latest areal and river
flood statements for more information.
On a positive note...the deep moisture that was present to allow the
storms to produce such heavy rain is shifting off to the east.
Additionally...even though some thunderstorms are expected this
afternoon...there looks to be enough upper level flow to keep them
moving to the east. Thus...not anticipating any widespread flooding.
Will need to keep the Flash Flood Watch going...though...because as
mentioned earlier the area has been so hard hit any rain will cause
a problem.
Additional rainfall is possible Tuesday through Wednesday
night...though Tuesday is trending drier for southeast Minnesota and
northeast Iowa. Hopefully the drier period from tonight through
Tuesday will allow the flood threat to diminish...though river
flooding may take a little longer to end.
&&
Arx watches/warnings/advisories...
WI...none.
Minnesota...Flash Flood Watch through this evening for mnz079-086-087-094-
095.
Iowa...Flash Flood Watch through this evening for iaz008.
&&
$$
Short term...aj
long term...aj
aviation...baumgardt
hydrology...aj
