Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
741 am CDT Monday may 20 2013
Short term...(today and tonight)
issued at 400 am CDT Monday may 20 2013
Today-tonight...broad upper level central Continental U.S. Trough will slowly
meander toward the east...with its cutoff low wobbling over western Minnesota and the
eastern Dakotas. Several shortwave spokes will rotate around this upper
low...swinging through the mpx County warning forecast area today into tonight. The main issues are
around timing of precipitation and potential for severe weather in far eastern Minnesota
into western WI. Scattered showers this morning will continue to develop and
dissipate as models indicate dry slotting to shift NE into the
region this morning...lasting through at least middle afternoon. This would halt
any widespread precipitation but isolated to scattered showers/thunderstorms certainly
cannot be ruled out. Once this dry slotting shifts well into northern Minnesota
and northwest WI by this evening...another swath of moisture will br dragged
into the region which will be ably tapped to produce additional
bands of precipitation...if not a large precipitation shield. One determining
factor will be the placement of the surface low under the upper low. The
surface low looks to remain primarily near the ND/SD/MN triple point...
and this position would keep the bulk of the precipitation along and north of
its latitude. As for severe weather potential...the eastward shift of the
upper trough puts the best jetting and exit region over far eastern Minnesota
and western WI. In addition...the break in the precipitation may allow for a
few breaks in the overcast...allowing for additional
instability...especially in western WI where precipitation has now virtually shut down
everywhere. This will allow a reloading of the atmos to generate
MLCAPE in the 2000-2500 j/kg range with little capping...while
central-western Minnesota will be dealing with less impressive lapse rates and
instability parameters. Still have a slight risk in place per Storm Prediction Center in
their swody1 for all of far eastern Minnesota through WI which is very reasonable.
As for temperatures and dewpoints...with the warm front continuing to
lumber northward through the County warning forecast area and its trailing cold front still in the
Dakotas...another warm/humid day can be expected. Will look for
highs from around 70 in western Minnesota to the lower 80s in western WI... with lows
tonight again mild in the middle 50s to lower 60s. Dewpoints will again
reach the lower 60s...keeping an unseasonably humid airmass in place.
Long term...(tuesday through sunday)
issued at 400 am CDT Monday may 20 2013
It isn't until Tuesday night/Wednesday morning that we finally
see some slow eastward progress with the trough over the central
Continental U.S.. during this time the surface low slowly fills and moves
from Minnesota to WI. However...before then...during the day on
Tuesday...we expect scattered showers across primarily central Minnesota
and northern WI. Just like this morning...where we see good drying
showing up in the water vapor imagery across eastern NE and
western Iowa...Tuesday should see a dry slot across the southern
half of the forecast area. Better q-vector convergence and
slightly better fgen is located across the northern forecast area
just north of the dry wedge shifting northeast during the day. As
a result...kept high chance probability of precipitation or likely probability of precipitation in the forecast
for the northern County Warning Area...but lowered probability of precipitation or went no weather in the
southern County Warning Area Tuesday/Tuesday night. Lingering instability in eastern Minnesota
and western WI warrants thunderstorms for the weather type on
Tuesday...but even before Tuesday night the MUCAPE slides farther
easy and diminishes. Therefore...kept the thunder out of the
forecast in most locations Tuesday night and Wednesday. We also
cooled temperatures a bit on Wednesday...especially across central
Minnesota...where light showers may be more widespread and hang around
longer. Also...with east-southeast shift to the system on
Wednesday...the better large-scale forcing should get pulled
across the area. So wednesday's probability of precipitation are actually higher than
tuesday's and are comparable to the probabilities forecasted
today. However...the quantitative precipitation forecast is low.
Looks like we'll get a brief reprieve from the wet weather Thursday
and Friday with a surface high building in trough...ridge...trough
pattern setting up across the Continental U.S.. the weather prediction center
prefers a GFS/gefs/ec/ec-mean blend in the extended and to that
seems reasonable given the relatively small spread in the long
wave pattern...even with an anomalously deep trough along the West
Coast. By the weekend...a warm front is forecast to move east
across South Dakota/NE and approach western Minnesota. Most of the forecast area
takes some time to moisten in light of the drier east-southeast flow under
the ridge. Nonetheless...with at least some moisture place...the warm
advection could kick off some showers by Saturday...especially in
western Minnesota.
&&
Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 711 am CDT Monday may 20 2013
Another day of radar monitoring and fairly frequent amendments.
Little bit of a tricky start with IFR/LIFR ceilings over much of the
area and determining any improvement and when. Main culprit is the
still-rotating expansive low pressure system centered over the eastern
Dakotas into western Minnesota. This system will slowly drift east today while its
associated warm front lifts north and the cold front lingers near the wrmn Minnesota
state line. Low clouds will gradually lift into MVFR range with
precipitation slowly coming to an end...especially for eastern Minnesota into western
WI. By this afternoon...enough daytime heating will allow convection to
develop across the entire region. Have held off timing to mainly
around 20z and afterwards. If/when any thunderstorms move across a terminal...
conds can easily be worse than what is advertised but did not want
to go too hard at this time. More showery precipitation is then expected
overnight with conds again degrading to at least MVFR tonight.
Kmsp...tricky start as conds have bounced between lower-end
MVFR... including below 1700 feet...and VFR at initialization. Have
covered this using a tempo group at the start and will monitor
trends to see if additional tweaks are needed. MVFR conds likely
to hold from midday Onward...with precipitation re-developing around
msp later this afternoon. Few thunderstorms at the start then more persistent
rains expected after 00z. Still some timing/duration complications
so some changes to the latter half of the taf would not be
unexpected. Winds start out southeast...become S...then become southeast again
during the early morning hours...and speeds will be gusty this afternoon.
/Outlook for kmsp/
Tuesday...VFR/MVFR ceilings. Rain showers/thunderstorms and rain likely. S-SW wind 5 kts.
Wednesday...VFR/MVFR ceilings. Rain showers likely. NE wind 5-10 kts.
Thursday...VFR. NE winds 10 kts.
&&
Hydrology...
issued at 249 am CDT Monday may 20 2013
Have maintained Flash Flood Watch as-is through this forecast
pkg but confidence is waning that actual flash flooding may be
achieved due to dry slotting moving into the region this morning. This
has cut into additional quantitative precipitation forecast expected. That said...much of the area
has received 2-4 inches of rain with locally higher amounts the past
couple of days...so soils are certainly close to if not completely
saturated. There may not be the quintessential flash flooding that
develops...but quick rises in rivers and streams along with urban
flooding due to runoff certainly cannot be ruled out as another one
half to one inch is expected today in much of the area covered by the
watch...especially in central Minnesota.
Even though the forecast is loaded with showers through Wednesday.
Today appears like the last remaining chance to get any kind of
quick response to local rivers and streams. We lose the
environment for vigorous deep convection Tuesday and Wednesday and
should just get scattered light showers with lower quantitative precipitation forecast totals on
both of those days.
&&
Mpx watches/warnings/advisories...
Minnesota...Flash Flood Watch until 7 PM CDT this evening for mnz041>045-
047>053-057>063-066>070-075>078-084-085-093.
WI...Flash Flood Watch until 7 PM CDT this evening for wiz014>016-
023>028.
&&
$$
Short term...jpc
long term...clf
aviation...jpc
hydrology...clf
