Scientific Forecaster Discussion
Area forecast discussion...corrected
National Weather Service Twin Cities/Chanhassen Minnesota
805 PM CDT Thursday Jul 24 2014
issued at 804 PM CDT Thursday Jul 24 2014
Only until recently have models begun to latch onto the current
situation. Hrrr and now hopwrf have begun to converge toward a
solution which allows convection over the western Dakotas and
eastern Montana to expand eastward rapidly during the late evening
and spill into Minnesota overnight. The global models through 18z
did not handle this idea well...so while confidence has been quite
low it is starting to increase. It remains to be seen whether the
00z global models will latch onto this solution...but given the
recent trends in the cams there is a pretty good chance.
Strong moisture advection over eastern South Dakota/Nebraska where
precipitable water values are 1.5 to 1.9 inches will continue to improve overnight
as a 40 to 50 knots low level jet develops. This low level jet will then veer eastward into
southwestern Minnesota overnight and focus this moisture transport over the mpx
County Warning Area. A strongly to extremely unstable airmass to the west should
maintain this developing mesoscale convective system as it continues east for the next 12
hours. Thus...the solution keeping activity confined to Iowa /a
la NAM...GFS...and CMC/ is looking less and less likely. Increased
probability of precipitation across central Minnesota into the likely range and the timing was
based on the latest hrrr/hopwrf.
Any severe threat will be determined by how far east the eml and
associated instability can progress. Best chance for any severe
would be across western Minnesota...closest to the eml. Large hail would be
the primary concern.
Short term...(this evening through friday)
issued at 233 PM CDT Thursday Jul 24 2014
Short term concerns remain coverage of thunder overnight into
Friday. Also...chance of large hail mainly later tonight into south
central Minnesota with some severe potential lingering into Friday ahead of
Upper ridge is beginning to move east with one impulse dropping
southeast over western Minnesota and the next wave lifting into
northwest nodak and southern Manitoba. It appears the initial wave
will drop southeast this evening along the eastern periphery of the
instability gradient to the west and the edge of low level warm air advection/steep
middle level lapse rates. Models diverge on development thereafter...
with several hires models indicating Canadian system takes over and
drops southeast overnight. Still some potential of low level jet driving
convection into mainly south central Minnesota late. Will carry higher
probability of precipitation to the south once again...in spirit with the previous
forecast. Should see some quantitative precipitation forecast move through the most of area.
Friday still see some potential of isolated/widely scattered
convection developing along the front as it moves across the eastern
portion of the area into the afternoon. Will hold onto some low end
probability of precipitation for this potential. Instability along the front does increase
during the day but low level convergence remains weak. Much of the
high end forcing is driven east during the morning as well.
Temperatures should warm into the middle 80s to the southwest with mix
with the weak front and upper 70s/lower 80s elsewhere.
Long term...(friday night through thursday)
issued at 233 PM CDT Thursday Jul 24 2014
Main concern in the extended period is whether rain showers/thunderstorms and rain develop
further southward across Minnesota than expected for late Sat afternoon/early
Sunday as a strong...and unusual upper low for this time of year
moves east-southeast across the northern plains...and into the western
Great Lakes by Sunday. Will follow previous forecast with the main
area of rain showers across central Minnesota/ec Minnesota and wc WI where both upper
level energy and higher relative humidity fields are evident. K-index and 850 mb
showalter index fields indicated mainly rain showers with some embedded
thunderstorms and rain for the typical diurnal time frame. Sunday morning/afternoon will
likely be breezy as the surface low deepens as it moves southeast across the
western Great Lakes in response to the upper jet.
No changes in the extended period beyond this weekend with both the
ec/GFS advertising a strong upper ridge building off the East
Coast of North America with a deepening long wave trough across
the eastern 1/3 of the Continental U.S.. this will keep our region cooler
than normal and mainly dry. Although cooler temperatures are
expected...Standard anomalies are not too impressive compared to
the deep southeast U.S. Where anomalies could drop to 2-3 blw normal.
For comparison...our region will be slightly cooler...but still
nice for late July/early August.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 630 PM CDT Thursday Jul 24 2014
Much to consider for this taf issuance. Decaying showers across
southern Minnesota have produced strong winds as recently as 23z at msp
and stp where gusts of 35 to 41 kts were reported. Gustiness will
subside this evening as the boundary layer stabilizes somewhat.
Strong moisture transport over the eastern Dakotas have produced
middle level clouds over western Minnesota at taf issuance time...and could
Foster development of rain showers this evening. Other ceilings...the MVFR
variety...have also developed northwest of msp along I-94. These
appear to be somewhat transient...so made mention of it for the
first hour in stc and axn tafs. Will need to watch for further
expansion. Overnight...models are all at odds with each other with
the potential next round of shra/tsra. The hrrr model seems to be
improving its handle on current activity over Iowa and eastern Montana...and
it shows a band of rain showers/thunderstorms and rain arriving between 06-07z over far western
Minnesota and heading east into western WI by 09z or 10z. This seems like the
most likely scenario at this point...but since so many other
models are showing little or no activity overnight was only
confident enough to mention a prob30 at this time.
Kmsp...the gusts have ceased following the unexpected 41 knots gust
at 23z. Mostly clear conditions are anticipated for the remainder
of the evening...although the MVFR ceilings to the northwest are a
little close to completely ignore the possibility of brief MVFR
conditions in the next couple hours. As mentioned in the general
discussion...confidence is low overnight for the next round of
rain showers/thunderstorms and rain so kept the taf at prob30. Come the 03z amend...should
have a better handle on the overnight forecast and be able to
remove completely or reintroduce prevailing.
/Outlook for kmsp/
Sat...VFR. Winds west-southwest at 5-10kts.
Sun...VFR. Chance rain showers/MVFR ceilings. Winds northwest at 10-15 kts.
Monday...VFR. Winds north 5 knots.