Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
739 am CDT Tuesday may 21 2013
Short term...(today and tonight)
issued at 328 am CDT Tuesday may 21 2013
The center of the surface low responsible for all of the sensible
weather the past few days continues to sit and spin /nearly
stationary/ across eastern South Dakota. Although there has been no
significant movement with the upper trough over the last 12
hours...the small 500 M height falls yesterday actually helped
deepen the surface low by 2-3 mb and perhaps even tighten up the
circulation and corresponding frontal boundaries across the area.
The attendant cold front stretching south across the central and
Southern Plains was responsible for focusing the violent weather
in the states to our south. But even this morning the stationary
boundary stretched out from west to east from the surface low
across central Minnesota and northern WI is the focus for widespread
showers and weak thunderstorms in these same areas. There is nice
middle level drying stretching from the Minnesota/WI border all the way
south into Kansas and OK. This drying will likely keep much of
southern Minnesota and portions of west central WI dry at least during
the morning. Closer to the middle level circulation in central Minnesota
/especially west central/ there should be light stratiform type
precipitation late this morning and afternoon where frontogenetical
forcing will be stronger. However...probably not a wise move to
completely remove probability of precipitation in the warm quadrant of the system across
southern Minnesota...because there should be enough afternoon instability
given decent bl moisture to pop some cumulus and afternoon showers.
There is still a decent thunder chance with any of the showers in
eastern and west central WI today. Available short-range WRF
guidance including The Hop-WRF /at hopwrf.Info/ support this.
Overall...the low fills and drifts southeast with time tonight.
There is still enough lift to warrant pop mention even with the
weakening trend...but do not expect heavy precipitation. Just pull up a
radar mosaic and look at the radars to our north
/kabr/kfgf/kdlh...those showers will continue to rotate
cyclonically around the low and when the low moves off to the
southeast...those showers will come along with it.
Long term...(wednesday through monday)
issued at 328 am CDT Tuesday may 21 2013
Cloudy...cool...and scattered to numerous light rain showers or
sprinkles are on tap Wednesday with the low still taking its time
to move across the forecast area. Really think by Tuesday night
the instability will be shoved far enough to the west that we
should just be dealing with the light showers and no thunder. We
are dry by early Thursday morning...but clouds should be slower to
clear. No major forecast changes in the Thursday-Friday time
frame. Went just a bit cooler with temperatures during this time.
The guidance trended that way...but simply think it is more to do
with slight changes in the timing of showers during the day on
Wednesday and clearing of clouds late Thursday night. This should
set the stage for a very cool Friday morning. Upper 30s are
certainly within reason across central Minnesota and west central WI with
the clouds moving out and that high moving in.
Medium to long range guidance continues to forecast troughs along
both west/east coasts and a ridge over the central states and a
closed anticyclone through the weekend into early next
week. Southerly flow ahead of the trough and surface low/warm front
development should focus precipitation over the Central Plains late in
the weekend and into the northern plains/mid-upper MS valley/Great
Lakes early next week where convection is likely as indicated in
the gefs/European model (ecmwf) ensemble members and GFS runs. There are a lot of
probability of precipitation in the extended...but it is difficult to remove them...even
given we have fairly low confidence in the timing and placement
of the showers and thunderstorms late this weekend and early next
week. Here's why its difficult to remove probability of precipitation. Climatology is
right around 25% for a 24 pop forecast this time of year. The
21.00z GFS has a longwave trough approaching from the west and
southwest flow here and the 21.00z European model (ecmwf) is less amplified and
even quasi zonal. Both of these patterns would be fairly active
and would certainly warrant keeping probability of precipitation at least at the
muted/averaged values of climatology.
&&
Aviation...(for the 12z tafs through 12z Wednesday morning)
issued at 655 am CDT Tuesday may 21 2013
Several complications with this taf set due to a wobbling warm front
over central Minnesota into western WI...including over nearly all taf sites.
This will make for varying wind directions...cloud heights and
timing for precipitation. As the fnt lifts north...mainly north through
krwf-kmsp-krnh-eau...winds will shift from east to SW over the next
few hours...especially as the cold front lingers over western Minnesota. North of the
warm front...kaxn-kstc will likely remain NE through the day. Have already
seen most sites reporting low-end MVFR to IFR. Although there will
generally be a lifting of the ceilings today...ceilings will mainly
remain within MVFR to low-end VFR. Scaled back the precipitation due to
lower confidence on coverage and the more showery nature of precipitation
rather than a solid rain shield. That said...there still may be
some isolated pockets of moderate-heavy rain that could impact visibility
including thunderstorms. Precipitation chances remain generally 30-60 percent through
the day...lowering this evening into tonight then low stratus likely
to develop overnight.
Kmsp...aside from patchy showers this morning which are not expeceted to
degrade visibility...will keep msp dry through the next 30 hours. Still may
see some passing showers during this time but nothing that should
drop visibility below 6sm. Will look for ceilings to rise above 3 kft
midday today before dropping back down this evening through tonight. Good
chance that ceilings will drop below the 1700 feet threshold overnight tonight
and remain there through the Wednesday morning push as the large upper level
low over western Minnesota shifts into WI. Winds will be tricky with the
wobbling front overhead followed by the upper level low. Winds are
expected to shift from east to SW by around noon...but the timing is
problematic to pin down and speeds are expected to remain at or below 7 knots
so have used vrb05 through the morning to get to the early afternoon wind
shift. Winds drop to vrb again tonight before picking up to northerly by
daybreak tmrw.
/Outlook for kmsp/
Wednesday...VFR/MVFR ceilings with -shra likely. Winds NE at 5g10kts.
Thursday...VFR. Winds north-northeast at 10-15kts.
Friday...VFR. Winds southeast at 5-15kts.
&&
Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&
$$
Short term...clf
long term...clf
aviation...jpc
