Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 221756
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
1156 AM MDT Mon May 22 2017

.UPDATE...
Issued at 1151 AM MDT Mon May 22 2017

Forecast looks on track. The main front is through Casper with
showers behind it. Wind pattern is complex with the winds turning
northerly and mixing stronger gusts down ahead of the temperature
gradient across southeast Wyoming. Still expecting some shower
development ahead of the front, but the main area of precipitation
will likely be in the temperature gradient which should move
through Denver around rush hour. Not too excited about the
strength of storms as convection should be fairly shallow with
CAPES under 500 j/kg. We could get some strong wind gusts with
outflow winds superimposed on the existing frontal winds, though
the complex wind field could take the edge off of the winds a bit
as well.

&&

.SHORT TERM...(Today and tonight)
Issued at 421 AM MDT Mon May 22 2017

Last night`s shortwave was indeed slower than earlier advertised,
and it`s still producing rain over the Palmer Divide as of 4 AM
this morning. Rest assured, this feature is now finally moving so
precipitation should be mostly done by 6-7 AM, except across
southern Lincoln County. A couple patches of fog possible north
of that where skies are clearing.

The current disturbance is getting a kick from our next weather
maker, which will be quickly arriving this afternoon. This
feature is already driving quickly southward across the northern
High Plains. 100 knot upper level jet is knifing straight south
across the spine of the Rockies, putting the forecast area in the
favorable left exit region of the upper jet late this afternoon
and evening. There is just weak Q-G forecast shown in the models,
but this pattern is known to produce widespread showers given the
upper level divergence, a neutral to slightly unstable airmass,
and passage of a strong cold front. Winds are expected to become
quite gusty behind the front this afternoon with gusts reaching 40
mph. Temperatures will drop a quick 15 degrees as well, possibly
more given cold outflow from showers and a few thunderstorms.

We have lowered snow levels into the late afternoon and evening
given the strong cold advection. Initially they will start out
quite high this afternoon with levels around 9-10 thousand feet,
but then lower behind the front to around 7000 feet. With this
blast, we should see good potential for a quick 2-5 inch snow
across the mountains, resulting in winter travel conditions
spreading from north to south 4-7 PM, and then lasting through the
rest of the evening as snow eventually tapers off. In the
foothills, would also expect some accumulation given the strong
cold advection, but amounts here will depend on how long
precipitation lingers behind the front. At this time think a
dusting to 3 inches is reasonable, but could still cause some
travel issues due to the intensity of convective showers. Finally,
on the plains, we will see showers and a few thunderstorms become
more numerous and spread south along and just behind the cold
front this afternoon. At this time, the front should reach the
northern border area by 2 PM and then across Denver toward 4 PM.
It should be noted these fronts can and often do arrive earlier
than forecast due to enhanced outflow from showers, so will
continue to watch that today. Overall, have increased PoPs and QPF
for this short wave disturbance through this evening given the
favorable parameters in play today.

Precipitation is expected to diminish rather quickly behind the
front later this evening with subsidence and drying building in.
We`re also losing the upslope forcing. Some clearing should occur
late, and with winds decreasing it will get quite cool tonight.
Patchy frost would not be out of the question if we get sufficient
clearing on the plains.

.LONG TERM...(Tuesday through Sunday)
Issued at 421 AM MDT Mon May 22 2017

Much of the next week is going to feature upper level ridging over
the west coast troughiness over Colorado. The northwesterly flow
aloft downstream from the ridge will contain shots of moisture and
embedded short wave impulses that will keep things unsettled
across the forecast area. The lone reprieve from the unsettled
pattern appears to be Wednesday when a short wave ridge pops up
over the state. Temperatures are expected to jump up that day as
warm air out of the desert southwest advects across the Great
Basin and Four corners region. Subsidence under the ridge and the
warm temperatures at mid levels should keep all convection over
northeast Colorado capped.

However, the upper pattern will be a progressive one with the
ridge sliding eastward as a short wave impulse rotates around an
upper low over Montana. A surface cold front will also move in
from the north with gusty winds and dropping temperatures.
Afternoon showers will then develop. Additional shots of energy
will continue rotating into Colorado into the weekend, with
temperatures remaining cooler than normal, and the chance for
afternoon and evening shower activity persisting. Snow levels in
the mountains will fluctuate between 7000 and 9000 feet. The cool
airmass will help to limit the strength of any convection.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1151 AM MDT Mon May 22 2017

North winds will increase this afternoon due to mixing, then a
cold front will move through between 21z and 00z bringing stronger
north winds and showers. Areas of MVFR conditions are likely,
along with a couple hours of wind gusts in the 30-40 knot range.
After 02z the showers will be past with higher ceilings, though
the higher cloud deck may still require instrument approaches to
KDEN late tonight into Tuesday.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Gimmestad
SHORT TERM...Barjenbruch
LONG TERM...Dankers
AVIATION...Gimmestad



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