Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
726 PM CDT Friday Aug 1 2014
Short term...(this evening through saturday)
issued at 305 PM CDT Friday Aug 1 2014
Scattered showers and thunderstorms will come to an end this evening
with the loss of daytime heating. Clouds scatter out tonight and
winds become light and variable...which may set the stage for
patchy fog development overnight...particularly in areas that
receive evening rainfall from central Minnesota into west central WI.
Saturday brings the return to southerly flow ahead of a northern
plains trough. 850 mb temperatures moderate to between +16 and +18c across Minnesota.
Should easily achieve middle 80s...with some upper 80s possible over
west central Minnesota. Lower 80s will be common across WI. The afternoon
will become more sketchy as far as being able to confidently
forecast dry weather...especially from west central WI into
southeastern Minnesota...where several models indicate a pocket of
instability /per convective available potential energy increasing to over 1500 j/kg/. There isn/T
any forcing mechanism to hang our hats on...but instability could
prove sufficient enough to generate isolated thunderstorms generally
along/east of Interstate 35.
The other area to monitor Saturday afternoon will be west central
Minnesota...as the cold front associated with the aforementioned trough
sags into Minnesota. While the main forcing will be displaced well
north of the area...could see a few isolated storms sneak into the
northwestern County Warning Area /near Alexandria/ during the late afternoon hours.
Certainly a higher probability as we head into Saturday night
Long term...(saturday night through friday)
issued at 305 PM CDT Friday Aug 1 2014
The main change in the extended period is to lower probability of precipitation by
midweek...especially across central Minnesota due to the continued
southerly trend of the frontal boundary and mean upper air flow
from the northwest.
The frontal boundary that is expected to slowly move southeast
across the upper Midwest by The Ladder part of the weekend should
continue low end chance probability of precipitation...especially across ec Minnesota/wc WI due to
better moisture pooling and upper level forcing. However...after
Sunday night this front may be far enough south to preclude
anything significant...especially in terms of heavier rainfall
potential as previously forecast had. This is based on the
stronger long wave trough across the eastern half of North America
which will lead to a more northwest flow in the upper Midwest. At the
surface...high pressure is expected to dominate with the best thermal
boundary SW of mpx County Warning Area.
There remains some uncertainty...especially in terms of the
strength of the long wave trough and the ec holding onto the
higher probability of precipitation Wednesday. However...am not confident on any long duration
rainfall event until the upper flow changes...and the higher precipitable water
values and associated disturbance across Baja California California...moves
over the upper ridge forecast to hold around New Mexico/Colorado and
Texas region through the end of next week.
In addition to the lower probability of precipitation and the front further to the S/SW of
mpx County Warning Area...thickness values allow for a cooler than normal week.
Aviation...(for the 00z tafs through 00z Saturday evening)
issued at 652 PM CDT Friday Aug 1 2014
Decaying thunderstorms sprinkled across the forecast area will
quickly disappear as daytime surface heating wains. Multiple
outflow boundaries from the aforementioned thunderstorms are
visible on radar and may cause sudden wind shifts through the next
few hours...but speeds will be light. Skies will be mostly clear
tonight with some high cloud cover spreading in from the
northwest. With clearing skies and light winds...fog will again be
a possibility at the prone locations. Conditions today were very
similar to yesterday and none of the taf sites saw any
appreciable rainfall from the isolated thunderstorms this
afternoon. With that in mind...the forecast for tomorrow morning
is similar with what occurred this morning...patchy fog in prone
areas. LIFR fog is still possible...but at this point I believe it
will be very isolated if it exists at all so confidence wasnt
there for adding it to the taf for now.
Kmsp...little concern this period at kmsp but starting out with
some isolated thunder in the vicinity of the airfield.
Otherwise...light winds clear skies expected overnight with winds
becoming more persistent tomorrow from the south and eventually
southwest...but with speeds of 5 or so knots. A frontal boundary
will be dropping south early Sunday but will not reach msp until
after this taf period.
/Outlook for kmsp/
Sat night...VFR. Slight chance thunderstorms and rain. Winds variable 5 kts.
Sun...VFR. Slight chance thunderstorms and rain. Winds SW 5-10 kts.
Monday...VFR. Chance thunderstorms and rain. Winds east-southeast 5-10 kts.
Tuesday...VFR. Chance MVFR in thunderstorms and rain. Winds east 5-7kts.