John can you translate this stuff for us lay people???

General Discussions: ASK John: John can you translate this stuff for us lay people???

By bryan_t on Sunday, November 29, 2009 - 11:48 pm: Edit Post

for us non weather nerds???...(lmao)

Below was written sunday evening by nws marquette, at a site I used to use for snow reports when I was a radio announcer (I.W.I.N.-or interactive weather information network)..I believe it's run by NOAA.

Not sure what all the "z"'s are about (is it zulu, time???) Here goes:

WEDNESDAY INTO THURSDAY...NW TO N FLOW CAA AND AN AMPLIFYING MID
LEVEL PATTERN WILL BRING IN SIGNIFICANTLY COLDER AIR WITH 850 MB
TEMPS DROPPING TO AROUND -11C BY 00Z/THU AND -BY 12Z/THU...PER
GFS/ECMWF...GIVING LAKE-H8 DELTA/T TO AROUND 18C. WITH DEEP MOISTURE
AND FAVORABLE INVERSION HEIGHTS AOA 7K FT...MODERATE TO HEAVY
ACCUMULATIONS ARE EXPECTED. THE GFS/ECMWF VEER WINDS FROM WNW TO NNW
WHICH WOULD IMPACT N CNTRL UPPER MI. HOWEVER...EXPECT LAKE INDUCED
TROF POSITION WOULD LIKELY FOCUS BETTER LES INTO W UPPER MI AND
LOCATIONS E OF MARQUETTE.

FRI-SUN...FCST LEANS MORE TOWARD THE ECMWF/CANADIAN SCENARIO WHICH
WAS LESS PROGRESSIVE RESULTING A LONGER PERIOD OF LES POTENTIAL.
SO...EXPECT NW FLOW LAKE EFFECT SNOW TO CONTINUE THROUGH SATURDAY
WITH WINDS GRADUALLY BACKING MORE WRLY FROM FRI INTO SAT. BY
SUN...SIGNIFICANT UNCERTAINTY WITH POTENTIAL FOR RIDGE MOVING IN AND
SHUTTING OFF LES. FOR NOW...WITH LITTLE SUPPORT FROM OTHER GLOBAL
MODELS OR GFS ENSEMBLE...DISREGARDED 12Z GFS SCENARIO OF SIGNIFICANT
WIDESPREAD SNOW INTO THE REGION.


By frnash on Monday, November 30, 2009 - 01:44 am: Edit Post

Let an amateur take a crack at the translation:


quote:

Wednesday into Thursday:
Northwest through northerly flow, cold air advection and an amplifying mid level pattern will bring in significantly colder air, with 850 MB level temperatures (around 5000 feet) dropping to around 11°C (12.2°F) by 00Z (Zulu) (7PM EST, 6PM CST) per GFS/ECMWF (Global Forecast System/European Centre for Medium-Range Weather Forecasts) computer models, giving lake H8 (lake level to 850MB level) temperature difference to around 18°C (32.4°F).

[Note: For lake effect snow, you need the temperature difference between the lake and the air at 850 MB (delta T) to be 13°C, or a difference in temperature of about 28-30°F between the lake and the air at about 2-3000 feet. The higher the Delta T, the heavier the snow. Next you need the proper wind direction to move the clouds and snow over the location of interest. A west northwest wind is best for the Keweenaw.

(For locations close to sea level, the 850 MB level approximates the top of the planetary boundary layer. In the planetary boundary layer, friction and turbulent motion eddies are common. The wind blows at a more constant speed and direction above the 850 MB level.) -FRN]


With deep moisture and favorable inversion heights at or above 7000 feet, moderate to heavy accumulations are expected.

[The more unstable the atmosphere the higher the clouds will form, the higher the clouds, the heavier the snow. An inversion height of 7000 feet will provide for more than ample cloud development. -FRN]

The GFS/ECMWF (Global Forecast System/European Centre for Medium-Range Weather Forecasts) computer models veer winds from west northwest to north northwest, which would impact north central upper Michigan.

However, expect lake induced trough position would likely focus better lake effect snow into west upper Michigan and locations east of Marquette.


Friday-Sunday:
Forecast leans more toward the ECMWF/Canadian (computer models) scenario, which was less progressive, resulting in a longer period of lake effect snow potential, so we expect northwest flow lake effect snow to continue through Saturday, with winds backing more westerly from Friday into Saturday. [More westerly would again favor the Keweenaw. -FRN]

By Sunday: Significant uncertainty, with potential for ridge moving in and shutting off lake effect snow.

For now, with little support from other global models or GFS ensemble, we have disregarded the 12Z (7AM EST, 6AM CST) GFS (computer model) scenario of significant widespread snow into the region.

[As John has said:
The dream scenario for lake effect snow for the Keweenaw is to have large lake/850 delta T's, on the order of 20°C or more. Winds out of the west northwest. An unstable atmosphere, with inversion highs of 7000 feet or more. Relatively moist air upwind of the lake. Uni-directional winds blowing at about 10-15 MPH or less. This would produce snows on the order of 1-3"/hr.]
In my amateur but never humble opinion, that spells serious LES! However, as for myself, I'm much more interested in seeing John's forecast on MONDAY! :-)


By admin on Monday, November 30, 2009 - 04:36 am: Edit Post

Nash handled the details, but it is pretty much the same as I wrote in layman's terms back on Friday:

"It also looks like temps will be getting cold enough that the lake snow guns will be brought into action by later Thursday into Friday and Saturday. Still a tricky call as to how much LES will occur, but with cold, moist and unstable air passing over waters that have had their temps propped up by the mild November, I would not be surprised at all to see some locales in the UP and maybe NWL MI pick up 12"+ of snow.

Things do look to quiet down in most of the region by later Saturday and the LES will likely taper off as we go through the day on Sunday."


Or more simply put, it looks conditions will be quite good for Lake Effect Snow starting up later Wednesday and continuing into Saturday.

-John


By bryan_t on Monday, November 30, 2009 - 09:56 am: Edit Post

Cool..thanks JOHN and Nash..looking good!!!


By lenny on Monday, November 30, 2009 - 06:46 pm: Edit Post

lot's of John's here


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