Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
405 PM CDT Wednesday may 6 2015
Short term...(this evening through thursday)
issued at 359 PM CDT Wednesday may 6 2015
Surface analysis depicts a warm front snaking southeastward
from east-central South Dakota through southern Minnesota into northern portions of the Ohio
Valley region. This is evidenced nicely on visible Sat imagery with
cloud streaks in a similar fashion with some clearing in the wake of
the warm front going S through Iowa. In addition...a narrow line of
showers/thunderstorms shows up on kmpx radar moving northward but with limited
lightning...and similarly limited quantitative precipitation forecast per kmpx rainfall totals.
Aloft...flow is generally S to SW with a kicker upper level trough axis
swinging northward from the Central Plains and middle-miss River Valley
region. The current precipitation is mainly due to a weaker upper level trough
disturbance and the warm front. Once this precipitation line clears...then
clouds will fill in over much of the area late this evening through the
overnight hours. As the larger trough axis to the SW approaches during
the pre-dawn hours...another round of more widespread rain/thunderstorms is
expected for the start of the day Thursday...lingering through at
least midday...before just lighter showers look to persist for much
of the remainder of the day. There is some concern that late-day
convection in advance of a cold front pushing eastward through the northern-Central
Plains may have enough clearing after the morning convection to
reload instability in the atmos ahead of the frontal lift. This may
allow for isolated strong/severe thunderstorm development in far southeastern portions
of the County warning forecast area late in the day. Chances are quite low...but Storm Prediction Center did
extend the marginal risk into southeastern Minnesota and southwestern WI for Thursday for a
hail/wind risk. Model quantitative precipitation forecast amounts have increased a bit last night through
today and this is reflected in the latest forecast thinking...more in the
one quarter to one half inch range County warning forecast area-wide although isolated
locations with strong storms/heavy rainfall could pick up upwards of
an inch through tmrw afternoon.
Long term...(thursday night through wednesday)
issued at 359 PM CDT Wednesday may 6 2015
Thursday evening through Saturday evening: the main focus for this
period will be precipitation chances through Friday morning as a
cold front moves eastward through Minnesota and WI.
A middle-level shortwave feature will move northeastward through Minnesota on
Thursday evening before moving into Ontario on Friday. The surface
low will already be in western Ontario by 00z on Thursday evening,
but a trailing cold front is expected to stretch southward through
central Minnesota at this time. Scattered strong thunderstorms...with
possibly an isolated severe storm...could persist ahead of the front
during the evening hours as cape values from 500-1500 j/kg and 0-6
km bulk shear of 20-30 kts are possible from 00z-03z over
southeastern Minnesota and west central WI. Combined with decent middle-level
lapse rates around 7 c/km and the frontal forcing...small hail and a
few strong wind gusts could occur over our southeastern counties.
The aforementioned parameters will diminish as the front moves off
to the east...and precipitation will gradually taper off through
Friday morning as forcing from the front and shortwave moves out of
Cooler temperatures will return behind the front from Friday
afternoon through Saturday...with highs in the lower 60s and lows in
the 40s. Dry conditions are expected throughout this period as high
pressure remains in place.
By Saturday night...a strong low pressure system is still exepcted
to be lifting northeast through Nebraska...with the surface low
likely reaching southern Minnesota by Sunday evening. Warm air advection regime
precipitation will likely begin spreading into southern Minnesota Sunday
morning...making for a cool and wet sunady through Tuesday morning
timeframe. It will mainly depart to the east later Tuesday but
upper level cyclonic flow may keep cloud cover and showers in our
area through that period.
Aviation...(for the 18z tafs through 18z Thursday afternoon)
issued at 119 PM CDT Wednesday may 6 2015
Complicated 06/18z taf set due to patches of low clouds being
quickly eroded from the S at the start then occasional periods of
showers/thunderstorms needing to be timed into and out of the terminals.
Started out trying to show improvement from S to north through the
terminals...going from LIFR/IFR to VFR...but there may well still
be some bands that race northward before dark. After a lull for much of
the overnight period...additional showers/thunderstorms look to develop from
the west and SW and gradually push across the area middle-morning through
early-afternoon Thursday. Have tempered thunderstorm prospects tmrw morning to a prob30
group rather than a tempo group due to higher uncertainty and
timing issues...but confidence is high enough to at least include
thunderstorm mention for tmrw.
Kmsp...waiting for msp to break out from the sub-1700ft cloud deck
at initialization...especially considering all surrounding sites
have already done so. Will see VFR conds for much of the evening
and overnight hours...aside from occasional bouts of showers/thunderstorms around
the 00z hour. More consistent rainfall with embedded thunderstorms looks
to develop during the pre-dawn hours then remain there for much of
the afternoon...and these may contain additional periods of MVFR or
even IFR conds if rainfall is heavy enough.
/Outlook for kmsp/
Friday...VFR. Winds northwest 10 kts.
Sat...VFR. Winds NE 10 kts.
Sun...MVFR likely with IFR possible in shwrs/tstms. Winds NE 10-15