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Area forecast discussion 
National Weather Service Denver/Boulder Colorado
337 PM MDT Friday Aug 22 2014

Short term...(this evening through saturday)
issued at 336 PM MDT Friday Aug 22 2014

Shortwave moving into western Colorado now with a large shield of
rain showers over the mountains. More stable air in the large rain
shield has limited lightning there...with stronger convection
around the edges. On the plains we wound up with a hybrid of the
leading edge of the lift and the east/west convergence zone.
Surface winds are northeast at 10-15 knots feeding into this
developing rainband. Another east/west band has developed near the
southern Colorado border and will also come north this evening
ahead of the main rain area. There has been some severe weather on
the east end of the convection in Kit Carson and Cheyenne
counties...threat should be lower further west but still enough
energy for some threat in our eastern tier of counties. As for the
heavy rain threat...everything seems to be moving. Some heavy rain
but backbuilding/training potential appears limited.

Slowed down the clearing in the midday update and will stick with
the existing forecast. It will take half the night for the main
area of rain to get past Denver...and there will likely still be
some weaker convection behind threat of morning fog is
looking low. If it did clear out nicely than there could be some
dense fog.

On Saturday there will be drying and warming aloft and it looks
like convection will be fairly well capped. Still expecting some
isolated showers and storms...most likely over the northern
mountains where temperatures aloft will be slightly cooler and
there may be a little lift from the wave passing north of US.

Long term...(saturday night through friday)
issued at 336 PM MDT Friday Aug 22 2014

A deepening upper level trough over the Pacific northwest is
expected to slightly increase the chance of showers and T-storms
across the forecast area Saturday night into Sunday. On Sunday...models
show the trough closing off over the northern rockies and a weak
cold front slipping south into northestern Colorado early Sunday
morning. 700-500mb spec humdities gradually rise to around 6g/kg
in northestern Colorado during the day Sunday. NAM and GFS show Post-
frontal east-southeasterly boundary layer layer developing during the day. GFS
generates an area of light quantitative precipitation forecast over and along the Front Range and
Cheyenne Ridge during the afternoon. European and Canadian models
are less optimistic about our rain chances on Sunday. All indicate
southwesterly steering winds of 20-30kts. So any storms should move at a
fairly good clip. Best shear and low-level moisture convergence is
prognosticated to move away from the northern Front Range and over the northestern
corner of the County Warning Area Sunday evening.

On Monday...this same southwest flow aloft appears to dry slot
the forecast area in the morning. Although later in the day...a second
wave of anomalously low heights associated with significant
cooling aloft swings southward out of the Pacific northwest. Jet
shear induced lift on the southeast side of this trough will
combine with Post-frontal low- level upslope flow on the plains
resulting in another shot of precipitation. However...Monday precipitable
water and specific humidity values lower than current values. So
precipitation amounts Monday afternoon and night from isolated to
scattered storm coverage not expected to be significant.

GFS shows the upper trough axis and main cold pool aloft passing
over the forecast area during the day Tuesday. While the 12z European model (ecmwf) and
Canadian Gem show this upper trough and cold air aloft passing
overhead Wednesday morning. That set...all three models show
cooler temperatures and a continued chance of showers/T-storms on
Tuesday...generally east of the mountains by mid-week...moisture and
cool air flush out with northwesterly flow aloft as a large high pressure
ridge building over the western Continental U.S..


Aviation...(for the tafs through 00z Saturday late afternoon)
issued at 336 PM MDT Friday Aug 22 2014

Scattered showers will increase later this afternoon with isolated
thunderstorms. There will be areas of MVFR ceilings and
visibilities this evening...with conditions lifting between 04z
and 08z. There is a slight chance of IFR/LIFR in fog early
Saturday morning...then VFR conditions are expected for the rest
of Saturday with only isolated thunderstorms in the afternoon.


issued at 336 PM MDT Friday Aug 22 2014

Some of the thunderstorms on the plains will produce heavy rain
through this evening. Fast cell motions will limit the flood
threat however. Training of cells would be required...and at this
time everything is moving. Most places will experience a period of
moderate to heavy rain as the band moves over...with 1 to 2 inches
of rain in 1 to 2 hours and expect that this will be mainly toward
the lower end of this range. Still some potential for heavier or
more prolonged rain...but the strongest convection and best
moisture will be shifting east of our area after the next couple
of hours. Further west there will be some significant
rainfall...but not as intense and the flood threat looks low even
in the burn areas.

Bou watches/warnings/advisories...


Short term...gimmestad
long term...Baker

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