Friday, August 30, 2013

OD Nats 2013 Aug 30 Weather

Friday forecast discussions are packed full of goodness, like a well balanced cereal.

Area forecast discussion 
National Weather Service Salt Lake City Utah 
311 am MDT Friday Aug 30 2013 


Synopsis...northern Utah will see a drying trend Friday through 
Sunday...with moisture returning to the area on Labor Day. Deep 
moisture will remain in place across the south through the Holiday 
weekend. 


&& 


Discussion...water vapor loop shows a ridge centered over The 
Rockies and a series of closed lows approaching the Pacific 
northwest. Several disturbances are marching westward across New 
Mexico and Arizona. Acars 400-250mb wind observations place a 
75-95kt cyclonically curved jet over the Pacific northwest. 
Rap...GOES...GPS sensors...and 00z slc radiosonde observation indicate precipitable 
water values range from 0.40"-0.70" eastern mountains to 1.10"-1.50" 
western valleys. 


Valley surface dewpoints are generally 45f-55f...closer to 60f along 
the Arizona border. Column moisture quite rich across the western 
valleys...maximized over utahs Dixie. With the ridge in 
place...believe Arizona disturbances will be kept south of the Utah 
border today and much of the convection will be significantly tied 
to the higher terrain. This is supported by the hrrr-3km simulated 
reflectivity. Storm motions are none which means if this is the case 
it will be difficult to get good coverage over adjacent valleys. But 
it also means if a storm were to develop over the adjacent 
valleys...it will sit and produce locally heavy rainfall with a 
flash flood threat. Have added mention locally heavy rainfall to 
southern zones but no watch at this time as coverage may be limited. 


With drier conditions and the ridge in place...kept probability of precipitation out of the 
forecast for the north today. It will remain quite hot given lack of 
convection and 700mb temperatures rising toward +16c. The record for 
number of 95f days in a Summer at Salt Lake City will be broken 
today. Speaking of records...the high min record for today at Salt 
Lake City is 71 set in 2012. This record is also in jeopardy 
depending on our morning lows. 


Global models indicate a series of disturbances with origins over 
the Gulf of Mexico will round the broad central Continental U.S. Ridge and 
impact the Great Basin this weekend. The combination of moisture 
transport and these disturbances should yield better coverage for 
convection especially the south. However...both GFS and European 
indicate in 850-500mb streamlines/specific humidity fields that a 
drier northerly flow may be present across the northern zones 
Saturday and somewhat Sunday. Slow storm propagation would favor 
flash flood threat Friday through Sunday across southern Utah if 
cloud cover remains across Arizona as currently depicted in guidance. 


Moisture surge is reinforced early next week with southerly return 
flow establishing with the ridge easing back to our east. GFS has 
been trying to bust a trough/closed low into the region for many 
runs and continues to back off as time approaches. Favor the 
European with a moist flattening ridge remaining the dominating 
feature over the region through the work week. Perhaps a more 
fall-like northwesterly flow establishing during the second week of 
September. 


&& 

You don't need to read all of it or even understand it all, but note the mention of drying trend today through tomorrow. This concerns me because I don't see that in the models.So is the forecaster speaking in general or relative terms? Hard to say... If you look at the precip potential from the NAM, you'll see what I mean. Looking at CAPE and LI values also indicate a lot instability with plenty of moisture on tap. It's hard to imaging today being less dry than yesterday, in fact I think it's the other way around... BUT I have been wrong on the timing of OD in the last 2 days, so perhaps I'm not quite calibrated.

Winds... Today we're looking at a near identical direction and speed as yesterday. It might even feel light and variable, but it is SSW and will be stronger around Ogden and more westerly out east of Evanston.

Top of lift looks a little higher than yesterday, at around 11-ish on the lower slopes of the wasatch and close to 15k deeper east, which is what cloudbase will be.

More later. Looks like another similar day.




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