(meteorobs) Weather Prospects for the Transit of Venus - VII

[Skywayinc at aol.com]

        I have just taken a look at the latest numerical output for both the 
NGM and ETA models through 48 hours.  The valid time at 48 hours is 8 a.m. 
EDT, or approximately one hour after the end of the transit.  The ETA is the 
preferred of the two models to use, because it's moisture field is depicted better 
than that of the NGM (which tends to be a bit too dry).  

       Basically, the cold front that has been mentioned here for the past 
few days is now expected to be relatively weak and not likely to penetrate much 
farther south than central New York and central New England by Monday evening. 
 It will then probably either wash-out . . . or move to the north as a warm 
front by Tuesday morning.

       On the ETA, moisture in three levels of the atmosphere are depicted.  
For 8 a.m. on Tuesday, the middle and upper levels (R2 and R3) of the 
atmosphere are forecast to be quite dry across much of the Northeast US, the Great 
Lakes States and Ohio Valley.  However, the lowest level -- the R1 -- is forecast 
to be very moist.  

       The R1 level depicts that part of the atmosphere between 1000 and 965 
mbs . . . or roughly between sea level and 1,500-feet.  

That is where most observers will likely be on Tuesday morning!  

       It has been my experience, that once the relative humidity in the R1 
level exceeds 80%, that scuddy low-level cloudiness tends to be prevailent.  
Obviously, the higher the humidity level, the thicker and more extensive is the 
cloudiness.  This type of cloudiness usually burns off/thins out as the Sun 
climbs higher into the sky.  

       Below I provide the relative humidity values for the R1 level for 20 
selected cities off of the latest ETA FOUS output.  As you can see, low 
cloudiness will be in issue at practically every location.  Meanwhile, above the R1 
level (above 1,500-feet) , the R2 and R3 levels have humidity values that are 
only about half as much!  

Location                               RH for R1 level
--------------                             ----------------------
Albany                                          86%
Burlington                                      76%
Boston                                          80%
Philadelphia                                   96%
Williamsport                                   97%
New York (La Guardia)                    92%
Portland, ME.                                 87%
Caribou                                          86%
Bangor                                           90%
Concord                                         78%
Bufallo                                           73%
Pittsburgh                                      76%
Cleveland                                       77%
Dayton                                           94%
Indianapolis                                    92%
Charleston, WV                              70%
Washington, DC                             97%
Norfolk                                           91%
Detroit                                            89%
Chicago                                         79%

       So . . . it appears that elevated areas would seem to have the best 
chance of success early Tuesday morning, while valleys and coastal locations 
stand a risk of getting "schmutzed-out" of this early-morning event.  Ironically, 
in all the cases cited below, the R1 humidity levels diminish rapidly after 8 
a.m. So Tuesday, for many locales could start cloudy and gray . . . burning 
off to a hazy/sunny midday and afternoon.  Of course, that would be too late 
for viewing the transit!

       One final note: a few people have asked me why their National Weather 
Service forecasts . . . or the forecasts on The Weather Channel are saying 
"Partly Cloudy"
for Monday night and Tuesday, with no mention of the potential cloud cover 
early on Tuesday morning.  The reason is, that 48 hours out, there is no need 
to
be so specific.  The forecasts issued on Monday, may mention "morning 
clouds" or "areas of fog" for the Tueday morning time frame.

-- joe rao






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