OD Nats 2013 Aug 31 Weather

Yesterday was no surprise to us when deep convection fired off in and around the area, starting early and not settling down for most of the night in some locations east. Given this trend coupled with the comments mentioned in the discussion below, we can expect a very active TS day, especially from Provo south. The north is a bit drier.

Area forecast discussion 
National Weather Service Salt Lake City Utah 
509 am MDT Sat Aug 31 2013 


Synopsis...a very moist airmass will remain in place across 
southern Utah today...while drier air remains across the far 
north. A weak weather system will bring this moisture northward 
Sunday into Labor Day. High pressure will redevelop over the 
region for the midweek period. 


&& 


Discussion...a broad middle level ridge encompasses much of the 
interior west this morning...with an embedded east-west oriented 
trough situated just south of the Utah/Arizona border. A very moist 
airmass is in place across southern Utah...with GOES derived precipitable water 
values at or above 1.25 inches...and values over 2 inches sitting 
just south of utahs Dixie along the Arizona/Nevada border. This moisture 
rich airmass combined with afternoon heating and the proximity to 
the aforementioned trough should allow for fairly widespread 
convection this afternoon and evening over southern Utah. A light 
easterly component to the flow aloft coupled with a tendency for 
storms to propagate into deeper moisture/instability should push 
this activity slowly downbasin across southern Utah...enhancing the 
flash flood threat. As such have issued a Flash Flood Watch for 
southwest and south central Utah for this afternoon and evening. 


Further north a ridge axis downstream from the aforementioned 
trough sits across northern Utah. A fairly sharp moisture gradient 
extends across northern Utah with the airmass remaining quite moist 
across areas south of I-80. Will likely see scattered convection 
this afternoon from roughly the southern oquirrhs southward into 
central Utah. Locally heavy rain will also be possible with this 
activity...although coverage will likely be more limited than 
areas further south. As such have left this area out of the watch 
for now. 


The middle level trough over northern Arizona is forecast to slowly lift 
northward across the forecast area beginning Sunday across 
southern Utah...and continuing into Monday over the north. As such 
anticipate another very active convective day over southern Utah 
Sunday afternoon...with the north becoming more active during the 
evening hours..and likely continuing overnight as this wave 
approaches. Have increased probability of precipitation over northern Utah 10-20 percent 
Sunday afternoon through Sunday night and into Monday morning. 


Thunderstorm coverage on Labor Day will be dependent on how 
quickly this wave moves northward...and the models at this point 
do not seem to have a firm grasp on this. Have maintained a chance 
of showers/thunderstorms into Monday afternoon over most areas 
with the highest probability of precipitation in the north. However...if this wave lifts 
into Idaho by early afternoon subsidence would likely inhibit 
additional convective development areawide. 


In the wake of this feature...the middle level ridge is forecast to 
strengthen across the region Tuesday into Wednesday. Sufficient 
moisture will remain to support a threat for showers and 
thunderstorms...however warming aloft will likely limit 
instability...with convective activity remaining largely confined 
to the higher terrain. An upper low which is forecast to reside 
off the Pacific coast during the first half of the week is 
forecast to move inland and through the northern rockies late in 
the upcoming week. This looks to suppress the ridge axis southward 
and allow for a drier westerly flow over northern Utah. 

Looks like a tough next 2 days ahead in terms of weather. So much moisture for being so late in the season around here!

See water vapor image here:



Winds aloft make the position of moisture a difficult prospect. Expect to see N to NW winds, albeit light, prevail across the region and strengthen as the day evolves. This makes heading north into the drier air mass very challenging, if not impossible. A quick look out the window this morning showed fairly large convection lit by the sunrise over the western deserts already. So expect to see another repeat of nuclear OD to the west. With a northerly flow, however, perhaps it will stay to the west? Hard to say.

Cloud base expected to be around 14-15k over the eastern and high wasatch terrain again. Yesterday did well with this prediction, so I suspect it will do well today.

More later.

-CG