Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
700 PM CDT Wednesday Jun 19 2013
Short term...(this evening through thursday)
issued at 413 PM CDT Wednesday Jun 19 2013
Surface ridging across the forecast area will fade this evening. This will
allow the low level moisture transport across the plains to begin
shifting into Minnesota during the overnight hours and eventually into west
central WI on Thursday. Conditions are still quite stable across the
County Warning Area at middle afternoon with the best Li still positive along with middle
level lapse rates below 6 degree c/km. Although the convective
temperature has been reached...still think that only isolated
activity will occur into early this evening. After that...the low
level Theta-E advection and middle level isentropic upglide begins to
increase. The WRF solutions are quite varied on what occurs
overnight and Thursday but the general consensus is that
showers/thunderstorms begin to sprout during the late evening over
eastern South Dakota and southwest Minnesota. This activity then gradually works east-northeast
during the overnight hours. The severe threat seems minimal at this
time.
On Thursday...there should be scattered showers and thunderstorms
ongoing in the morning across all of the County Warning Area. The activity should
diminish across west central Minnesota by afternoon as rather warm
middle level air begins to spread in. However... the showers and
thunderstorms may fester through the afternoon across east central
and south central Minnesota... as well as west central WI... in the zone of
the strongest low level wind field. Again...the threat of severe
seems to be confined to south central Minnesota during the afternoon hours
with a more widespread risk of severe weather spreading in from the
Dakotas Thursday night.
Temperatures this afternoon were certainly more Summer-like with
upper 80s to around 90 being achieved across west central Minnesota. Highs
on Thursday are forecast to be more in the lower to middle 80s
across the forecast area due to more clouds/shower risk...but certainly the
potential is there for it being a little bit warmer if the
convective trends do not pan out.
Long term...(thursday night through wednesday)
issued at 413 PM CDT Wednesday Jun 19 2013
Confidence is increasing that several rounds of convection will
traverse the region over the next few days bringing severe weather
and heavy rain. A low amplitude upper level ridge currently overhead
will shift over the eastern Continental U.S....while an upper level low
currently centered over Puget Sound sound will gradually slide eastward
across the High Plains of Montana and the western Dakotas. Middle/upper
level southwest flow will set up across the upper Mississippi River
valley with embedded shortwaves periodically triggering
afternoon/evening showers and thunderstorms that will grow upscale
into an mesoscale convective system and propagate eastward overnight as the low level jet
veers. Meanwhile an elevated mixed layer will form daily over the
foothills of Colorado. This airmass will advect across the region in
the southwest flow. At the surface a trough of low pressure will
remain nearly stationary over the eastern Dakotas. A subtropical
high will settle across the southeastern plains...and the
combination of the two will result in southerly flow supporting a
continuous supply of low level moisture from the Gulf.
The combination of the warm moist air at the surface...and elevated
dry air aloft will create cape profiles of 2000-3000j/kg across
Minnesota and Wisconsin. The sref 19.15 MUCAPE probabilities of
2000j/kg are in the 90 percent across southern Minnesota from
Thursday evening through the end of the model run Saturday evening.
The elevated mixed layer should cap off the thunderstorm development
by convective temperatures alone. However...a convergence along a
frontal boundary that is forecast to lay out across the upper
Midwest will be sufficient to break through the cap by late
afternoon. In addition...the low level jet will ramp up like
clockwork every night after sunset and these storms will tap into
the elevated instability and develop into an mesoscale convective system. Bulk shear will be
30-40kts...so organized convection is expected with mainly a wind
and hail threat overnight. One concern is the potential for
tornadoes with the veering wind profile along the frontal
boundary...provided storms are able to form along this boundary and
become surface based. Storm Prediction Center has slight risk both Thursday night and
Friday...but as alluded to in their discussions there is still some
uncertainty with the mesoscale features that determine convective
Mode.
By the weekend forecast models show the frontal boundary with an
east/west orientation across central Minnesota into Wisconsin. The
GFS and Gem are still farther north than the European model (ecmwf)...but the GFS
19.12 has trended south when compared to previous runs. It also
still has a large swath of 5+ inches of quantitative precipitation forecast...while the European model (ecmwf) and
Gem are more in the 3-4 inch range over a slightly broader area. In
summation...there is concern for flooding from heavy rain...with
more details regarding this in the Hydro section of the afd. Looking
ahead...according to the GFS 19.12 this upper level low finally moves
into Ontario early next week...but another wave will follow quickly
on its heels keeping rain chances through at least Wednesday.
Meanwhile the European model (ecmwf) tries to build a ridge across the Midwest...but
timing of this features keeps getting pushed back with subsequent
model runs so will not buy into a drying trend anytime soon.
&&
Aviation...(for the 00z tafs through 00z Thursday evening)
issued at 629 PM CDT Wednesday Jun 19 2013
A lot of uncertainty concerns with this set of tafs...mainly
regarding thunderstorm coverage and timing not only tonight but
through Thursday. A few storms have developed early this evening
near the South Dakota/Minnesota border. Expecting coverage to increase to the north
this evening...then eventually work eastward along I-94 late
tonight and early Thursday. There could be a break...especially
across western Minnesota Thursday morning and early afternoon before another
round develops over the Dakotas and spills southeastward toward
the end of the period. Confidence for more continuous rain showers and
thunderstorms and rain Thursday will be across western WI. Maintained the basic idea
from the last taf issuance by leaving out any prevailing thunderstorms and rain
until confidence increases...hopefully later this evening.
Kmsp...will be dry into the early overnight. Best estimate on
arrival of rain showers and possibly thunderstorms and rain will be after 09z tonight and
before 14z Thursday. Could be some MVFR conditions with any
activity. May be a few showers or storms lingering around eastern Minnesota
Thursday...before a healthier round arrives toward the end of the
taf period.
/Outlook for kmsp/
Thursday night...VFR...with MVFR/IFR and thunderstorms and rain likely at times. Southeast wind
10 kts.
Friday...VFR. Chance of MVFR/tsra. Southeast winds 10 kts.
Sat...VFR. Chance of MVFR/tsra. Southeast winds 8 kts.
Sun...VFR. Chance of MVFR/tsra. S winds 10 kts.
&&
Hydrology...
issued at 413 PM CDT Wednesday Jun 19 2013
A wet pattern is expected to develop across the upper Mississippi
River valley region from late this week into early next week with
flooding potential. A very slow moving upper air pattern will allow
for several rounds of showers and thunderstorms late this week...
this weekend and even into early next week. Moisture levels will be
on the rise by Friday and the warm and humid air mass will remain
entrenched across the region into Monday. Precipitable water values are prognosticated to
better than 1.75 inches through the weekend...which is more than 2
Standard deviations above normal. Ffg values are quite low with 1
hour values around 1.5 inches across the Minnesota River Valley and south
central Minnesota. Three hour values in these same areas are just under 2
inches. Areas that experience thunderstorms...especially this
weekend...will likely have rainfall rates approaching 2 inches an
hour. Hence...it will not take much for localized flooding to occur.
&&
Mpx watches/warnings/advisories...
Minnesota...none.
WI...none.
&&
$$
Short term...rah
long term...jrb
aviation...borghoff
hydrology...rah
