Area forecast discussion
National Weather Service Glasgow Montana
553 am MDT Monday may 20 2013
Short term...for northeast Montana...today through Wednesday...
6am update...the western push of rain looks to fall short of
Glasgow therefore backed down the 100 probability of precipitation. Proton
Today and tonight...a vertically stacked low pressure system
situated over the Dakotas will continue to sit and spin. As it
does...a shield of stratiform rain will continue to retrograde
from east to west though the day...before crashing back to the
southeast this afternoon and evening as the low begins to
translate eastward. Determining the maximum extent of the back
edge of the rainfall shield will continue to be the main forecast
challenge.
The latest NAM had the precipitation initialized a bit further to
the east than radar observations showed. Thus...feel the
precipitation shield should make it a bit further than this
solution is suggesting. The 00z European model (ecmwf) and GFS have a little better
handle on it. Will admit that the hrrr has been an impressive
informer as it caught onto the idea that the shield would stop
retrograding for a few hours overnight before again proceeding its
March further west toward morning and this is what is occurring.
Lending further support to the guidance is the observation of
recent pressure falls across the County Warning Area. Pressures are generally
falling between 0.5 and 1 mb per hour for the last few hours.
Therefore...will feel comfortable with a blend of the
European model (ecmwf)/GFS/hrrr model solutions for determining a best guess as to
where the back edge will reach before it begins to push back to
the southeast. This brings it to a line from about Opheim...extending
south through just west of Glasgow...to The Jordan area. To the
east of this line...there is a decent chance at a nice wetting
rain through tonight. In fact...would not be surprised if some
areas see one to two inches of rainfall accumulation by the end of
tonight and included a mention of heavy rain in some of the
eastern zones through the morning. To the west of this line...rain
chances will only be showery in nature with a few hundredths of an
inch of accumulation the absolute most that would occur. Precise
rainfall amounts will depend upon the precise location of where
the back edge of this rain shield winds up...as well as some finer
mesoscale details that are difficult to determine with much
confidence. Temperatures are kept around or just below seasonal
normals due to the presence of the lingering upper trough.
The other concern today will be strong gusty winds due to a sharp
pressure gradient in association with the surface low across the
Dakotas. Am expecting that the tight packing of the isobars could
support northerly winds on the order of 20 to 30 miles per hour with some
gusts exceeding 40 miles per hour this afternoon and evening. Will hoist a
lake Wind Advisory from noon until 9pm due to this concern. This
may also lead to wind driven rains in some locations which could
pose as hazards to travelers and pedestrians alike.
Tonight through Wednesday...the low pressure system over the
Dakotas very gradually translates eastward...so continued to trend
probability of precipitation down and have precipitation shutting off from west to east.
Will leave only a slight chance of rain showers in the extreme southeast
zones by late Tuesday night. On Wednesday...another upper trough
with closed low moves into the Pacific northwest as it does
so...diffluent southwest flow aloft is expected to develop across
the County Warning Area. This may lead to convective showers on Wednesday...mainly
across the SW zones. Some of the soundings in the NAM BUFKIT just
west of the County Warning Area are showing total totals approaching 50 and Li
becoming negative. Did not insert a mention of thunder just yet due to the
fact that the primary focus was on the current rainfall situation
and lake wind potential. However...future shifts may want to
evaluate the potential for any convective initiation toward the
middle of the week. Maliawco
Long term...Wednesday night through Monday...
the extended features more southwest flow through the period. Most
days slight chances of thunderstorms sliding off the central
Montana Mountains into the plains for higher probability of precipitation west and low
probability of precipitation east. For the weekend the upper low breaks a good wave off
and drives it through northeast Montana for a better chance of
thunderstorms therefore bumped up probability of precipitation a tad for those periods.
Temperatures will hold in near normal through the period.
Model trends are showing lowering heights on the GFS for northeast
Montana which leads to the upper low moving towards the east. If
the trend continues then likely probability of precipitation are a good bet for the
weekend with elevated confidence. Rmop/ensembles are showing that
its a good bet that at some point this upcoming weekend for a
strong thunderstorm event across eastern Montana. Proton
&&
Aviation...
MVFR for the eastern areas including ksdy and kgdv through the
period as they are closer to the low over southeastern North
Dakota which is wrapping around moisture into the lower Yellowstone
River Valley. This trend looks to continue through the period.
VFR for western areas including kggw and kolf through the period
as the MVFR ceiling shield looks to stay just off to the east. For
now will stay with VFR ceilings.
Wind will be more of an issue for the eastern sites with the winds
staying up above 20 knots through the end of the taf period. Proton
&&
Hydrology...
the next couple of days will continue to be very wet as northeast
Montana remains under the influence of a large upper trough and
vertically stacked low pressure system situated across the
Dakotas. A large area of stratiform rain will affect especially eastern
areas of the forecast area. Some locations could pick up between
one and two inches of rainfall by Tuesday morning...especially in
eastern Roosevelt and Richland counties where the steadiest rain
is expected.
Most of the basins in northeast Montana will be able to
accommodate this amount of moisture. However there is some
concern for rapidly rising streams...field flooding and localized
flooding in towns as the event persists. Some minor flooding may
become an issue...especially for the areas from Wolf Point to
Circle and eastward into North Dakota where the potential for rain
is the highest.
&&
Glasgow watches/warnings/advisories...
lake Wind Advisory from noon today to 9 PM MDT this evening for
central and southeast Phillips...central and southern valley...
Garfield...McCone...Petroleum.
&&
$$
Weather.Gov/Glasgow
