Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Twin Cities/Chanhassen Minnesota
642 am CDT Monday may 4 2015
Short term...(today and tonight)
issued at 345 am CDT Monday may 4 2015
The surface front and accompanying showers and thunderstorms have
sunk south into Iowa and Illinois overnight. Skies have cleared
across much of the forecast area early this morning. The main
questions for the short term are if...when and to where this
boundary will lift back north during the period.
The 00z GFS...ECMWF...and Canadian keep the front way to the south
through not only this period...but into Tuesday evening and thus
have shown a much drier solution through that time. The 00z
NAM...WRF-nmm and WRF-arw and to some degree the hopwrf members and
rap are more aggressive with the northern propagation of a middle level
wave from the southwest. This results in rain lifting north across
Iowa into southern Minnesota this evening and central Minnesota overnight. The 850 mb
jet on the 00z NAM would support the more northerly solution as its
nose sets up over northern Iowa. However...there are some potential issues
with this solution. First...the building upper ridge over the east
steering this middle level disturbance may not build as far north as
prognosticated given the trough to its north traversing east across the
northern Great Lakes. Second...considerable thunderstorm activity
will develop along the front and will very likely steer the low level jet to
the east...not north as the 00z NAM would indicate. After much
collaboration between abr...fsd...dmx...and arx...decided to reduce
probability of precipitation on the northern periphery of the potential complex to account
for a more southerly shift. Tough to go against the cams at
times...but the overall pattern would suggest this is a better
course to take. The good news is as this afd is being typed...the
06z NAM has shifted south closer to the GFS/ECMWF/Canadian solution.
Of course best laid plans...
Should see middle level clouds increase today but they may decrease
again across central Minnesota tonight. Highs today should reach near 70
area wide. Temperatures may drop into the upper 30s in some places of
central Minnesota tonight under the clearing skies and with dry air in
Long term...(tuesday through sunday)
issued at 345 am CDT Monday may 4 2015
The longer term concerns remain timing of short wave troughs
lifting northeast in the developing southwest flow aloft as
western Continental U.S. Trough translates east. This will provide a
generally wet forecast with probability of precipitation in generally every day in the
period. I tried to delineate best chance for a dry period(6 hours or
more) based on model trends.
Initially the frontal boundary over Iowa will gradually lift
north as a warm front...Tuesday night and Wednesday. The initial
dry atmosphere will eventually saturate as moisture lifts north
into southern Minnesota. This will spread a chance of showers and
isolated thunder into mainly the southern half of the area on
Tuesday. The first short wave trough lifts northeast into the
area Wednesday into Thursday. This should spread deeper moisture
and directed forcing/lift across the area. Will continue the
likely pop trend into the west Wednesday and over most of the County Warning Area
The GFS and European model (ecmwf) differ a bit more after this period...with the
European model (ecmwf) faster with moving the front through the area
Thursday/Thursday night. The GFS stalls the front over the east
into Friday as the flow aloft remains more southwest. Will try
and limit probability of precipitation over the east for Friday/Friday night. Timing of
the next shot of energy/upper trough in question in later
periods...as the East Coast ridge may be affected by the orphan
cutoff low that the models have been developing the last few
days. This may slow the troughs progression east. Model trends
have been lifting this into the region ,ate in the period...mainly
Sunday/Monday period. Will hold onto model guidance of likely probability of precipitation
With the general cloudy period developing this week...temperatures
are expected to range slightly above normal through midweek...and
then cooling with passage of the cold front Thursday...lingering
into the weekend. The quantitative precipitation forecast forecast through Thursday has 0.50 to
0.80 of much needed rain across the County Warning Area. The best chances for
higher end quantitative precipitation forecast come Wednesday night through Thursday.
Aviation...(for the 12z tafs through 12z Tuesday morning)
issued at 642 am CDT Monday may 4 2015
VFR conditions expected through the period. Models have trended
slower with the front returning back to the north...and expect
rain to remain confined to Iowa through tonight. North winds will
veer easterly late today and tonight as high pressure pushes east
over the Great Lakes.
Kmsp...no additional concerns.
/Outlook for kmsp/
Tuesday...VFR early. Chance rain showers/thunderstorms and rain afternoon/evening with MVFR/IFR conds.
Winds southeast 10-15 kts.
Wednesday...VFR. Chance MVFR/IFR conds in shra/tsra. Winds southeast 10-20 kts.
Thursday...MVFR conds likely with IFR conds possible in shra/tsra.
Winds S 10-20 kts.