Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Grand Forks ND
932 am CST sun Dec 21 2014
issued at 932 am CST sun Dec 21 2014
There was some clearing overnight west of the kdvl and Langdon
areas. Temperatures took a nose dive in those areas while everywhere
else stayed in the low 30s. Flow has remained southerly and there
is still no sign of anything to scour out the low level moisture.
Therefore will go ahead and keep the areas of fog around today
too. The lowest visibilities remain along the clear/cloudy line almost
exactly where they were yesterday...Langdon to kdvl to kjms and
south from there. Several sites along this line are again down to
about a quarter mile visibility. The lowest visibilities seem isolated at this
point so will continue to monitor a while longer to see if they
improve...otherwise may consider a dense fog advisory again. There
are still some remnant snow showers around the Lake of The Woods
which should move east by 18z or so. Rest of the afternoon will
remain cloudy and foggy with fairly steady temperatures.
Short term...(today through monday)
issued at 325 am CST sun Dec 21 2014
Main forecast challenge revolves around upcoming pre Christmas
snow event and expect snow amounts. Confidence rising as models coming
into better agreement and will use model blend and National
guidance for anticipated snow amounts.
Area of mainly -sn associated with current wave and zone of
isentropic lift will continue to propagate east and will monitor
for morning probability of precipitation. Once this moves through this am remainder of the
day should be mainly dry with persistent fog and low ceilings. With
southerly flow...continued warm advection and warmer start most of
the forecast area will be a few degrees either side of 32f.
Wave will drop into deepening trough tonight closing off over
South Dakota/NE region by morning. Surface inverted trough from South Dakota surface
low gradually takes shape across the central Dakotas. Lowering
condensation pressure deficits and isentropic lift confined to the
southern forecast area so have highest probability of precipitation there towards morning. Clouds will
hold temperatures up overnight.
Stacked low drifts slowly southeast Monday allowing surface inverted trough
to shift into the forecast area. Via Omega fields and 290k isentropic surface
lift increases during the day and raised probability of precipitation accordingly. Have
highest probability of precipitation associated in zone of 850mb frontogenetic forcing
although at this point banding snow potential does not look great.
Also from the 290k surface full saturation looks spotty. Based on
surface temperatures expect close to a 10:1 snow ratio.
Long term...(monday night through saturday)
issued at 325 am CST sun Dec 21 2014
Lift looks maximized Monday night and should be the favored period
for accumulating snow. Fortunately wind does not look to be a
serious issue although could see some drifting in open areas.
Low begins to fill Tuesday and isentropic lift diminishes during
the day. Still expect -sn during during the day but should
diminish from west to east during the evening. Cold advection weak
and mixed layer very shallow so winds should be manageable.
Considering headlines...based on model solutions and National
guidance feel this will be a fairly extended period of mainly
light snow. With wind also not appearing as a serious issue do not
feel watch warranted at this point although future advisories
Wednesday-Saturday...a bit of light snow could linger east of the
valley on Wednesday as the low continues to lift away from the area.
Thereafter have maintained a primarily dry forecast. The 00z
GFS/European model (ecmwf) indicate a more progressive Pacific shortwave trough
crossing the central/northern plains on Christmas...with precipitation
suppressed mainly south of the region. There will also be a few
shortwave troughs dropping southeastward in amplifying northwest
flow aloft. While any of these may produce some light snow...they
would be rather minor. Perhaps more importantly...this pattern will
allow colder air to filter southward for the latter half of the
period. The GFS/European model (ecmwf) both indicate 850 mb temperatures dropping to -20c by
next Saturday...bringing to an end our recent string of near to
above average temperatures.
Aviation...(for the 12z tafs through 12z Monday morning)
issued at 652 am CST sun Dec 21 2014
Dvl broke out of the low stratus overnight however stratus looks
to oozing back to the west so this site will need to be monitored.
Otherwise only slight improvement expected today and kept taf site
ceilings in IFR/or low MVFR range through the period. Lower visibilities
spotty with most sites from 3-5 miles in br. Could see some
improvement this afternoon.