Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
550 am CDT Wednesday may 27 2015
Short term...(today and tonight)
issued at 400 am CDT Wednesday may 27 2015
Today and tonight...early morning satellite imagery indicated a
compact circulation advancing northeast through northwestern WI. The
back edge of the cloud deck associated with this feature was pushing
into extreme western Minnesota...a trend of clearing skies expected to
progress eastward through the day. The concern then becomes if any
cumulus and possibly elevated popcorn showers/thunderstorms develop this
afternoon. Model soundings indicate deep mixing to near 750mb and
the maximum T forecast was based off a mix down from this level...and is
slightly cooler than the previous forecast...in part due to the
widespread rain yesterday moistening the surface.
Northwestern WI still looks to have the best shot at thunder
today...still in the cyclonic circulation departing over the
northern Great Lakes. However...evaluation of several hi-res
convective allowing models indicate decent agreement in bringing
isolated thunder through the entire forecast area today. The hrrr is the most
concerning as every new hourly run that has come in overnight
continues to consistently show scattered showers and thunder
developing during the afternoon. This makes sense given model
soundings indicate that the convective temperature in the low 80s
will be reached and show instability in the 700-300mb layer. I
Don/T feel confident in thunder actually developing given the
ridge pushing in...but given that the
hrrr/arw/nmm/mpxwrf/hopwrf/NSSL WRF all are trying to generate
popcorn showers and thunderstorms...decided it was best to include
isolated thunder across the remainder of the area for just a few
hours during the afternoon for diurnally driven convection.
We'll quickly get back in southwesterly flow aloft tonight on the
back side of the upper level ridge...which is a precursor of things
to come on Thursday and Friday. A shortwave will advance into the
Dakotas tomorrow night and will spread cloud cover into the western
half of the forecast area by Thursday morning. Guidance generally agrees the
precipitation will be just about to arrive early Thursday morning...so kept
slight chance probability of precipitation in for far western Minnesota early Thursday morning
ahead of this next system...which will be discussed in more detail
in the long term portion of this discussion.
Long term...(thursday through tuesday)
issued at 400 am CDT Wednesday may 27 2015
More showers and thunderstorms are in store for the region during
the early portions of the long term. Deterministic solutions are
in good agreement on deepening a northern stream short wave as it
heads from the northern plains through the upper Mississippi
Valley from Thursday through Friday. The NAM is the most
aggressive with a closed 700 mb low forming over eastern NE and Iowa on
Friday. All of the solutions point to a strengthening upper jet
over southern Ontario with the right entrance region lying across
northern Minnesota. In addition...rather strong middle level frontogenesis
will pass across the forecast area on Friday. The main concern during this
period is the potential for heavy rain with precipitable water
values prognosticated to reach around 1.6 inches on both the NAM and
GFS. This is in record territory for mpx sounding climatology for
the end of may. In fact...the maximum moving average is 1.54 inches.
Hence...likely probability of precipitation begin across western Minnesota Thursday afternoon and
transition east during the night. Likely to categorical probability of precipitation are
in place for Friday...especially from the Twin Cities on east and
south...when the best forcing moves through. Widespread rain
amounts of 1.25 to 1.5 inches are expected from the Twin Cities on
east and south with amounts from 0.75 to 1.25 inches to the west
and north. A few strong to severe storms are possible late
Thursday from west central into central Minnesota ahead of the
approaching cold front. The severe threat on Friday looks to be
south of here due to the extensive clouds/rain across the forecast area.
Saturday will be a sunny but cool day as high pressure dips down
across the area from the north. The high will move off to the east
rather quickly on Sunday with southerly flow increasing ahead of
the next low pressure system moving into the northern plains. This
feature may bring some showers and thunderstorms to the forecast area for
Monday and Tuesday. This would be the idea from the GFS while the
European model (ecmwf) would spread the activity northeastward from the Dakotas
into northern Minnesota...keeping the forecast area dry. At this point...only low
chance probability of precipitation are in place.
The warmest day in the long term is Thursday with highs around 80.
High temperatures drop about 10 degrees for Friday (70) and
another 10 degrees for Saturday (60). Temperatures begin a slow
climb with middle to upper 60s on Sunday...upper 60s to lower 70s
for Monday...with middle to upper 70s for Tuesday.
Aviation...(for the 12z tafs through 12z Thursday morning)
issued at 539 am CDT Wednesday may 27 2015
After morning fog and low ceilings in western WI and eastern Minnesota
improve in the next 2-3 hours...expect predominantly VFR
conditions for the period. Will have to monitor potential for
showers and thunderstorm development today based on hi-res
guidance and the presence of weak cape. Currently expect any
convection to be isolated so did not include at any taf site in
Minnesota. In WI however...did include vcsh as chances of shower and
thunderstorms are better there. Clouds will be on the increase in
western Minnesota at the very end of the period ahead of the next wave.
Kmsp...best period for isolated thunder will be from 20z-00z this
afternoon...otherwise expect VFR once this morning fog lifts.
/Outlook for kmsp/
Thursday...VFR. Chance afternoon -shra/-tsra. Winds S 10-15 kts.
Friday...MVFR/IFR ceilings/visibilities in shra/tsra. Winds SW 5-10 kts becoming
north-northwest at 10-15 kts.
Sat...VFR. Winds NE 10-15 kts.