Travel problems tonight for Nebraska

[senorpepr]

Looks like a nasty night/morning ahead for anyone driving back to work Monday morning. Although freezing precip chances are, in my opinion, rather slim.

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URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE OMAHA/VALLEY NE
1018 PM CST SUN NOV 26 2006

.A FAST MOVING STORM SYSTEM WILL BRING LIGHT FREEZING RAIN SHOWERS
ACROSS THE AREA FOR THE REMAINDER OF THE NIGHT. RAIN FALLING INTO
THE COLD ARCTIC AIR MASS AT THE SURFACE WILL CAUSE PRECIPITATION
TO FREEZE ON IMPACT OF SURFACES. A BULK OF THE PRECIPITATION
SHOULD MOVE THROUGH THE AREA EARLY THIS MORNING.

IAZ043-NEZ011-012-015>018-030>033-045-052-053-066-067-078-271230-
/O.CON.KOAX.ZR.Y.0001.000000T0000Z-061127T1400Z/
MONONA-KNOX-CEDAR-THURSTON-ANTELOPE-PIERCE-WAYNE-BOONE-MADISON-
STANTON-CUMING-WASHINGTON-DOUGLAS-SARPY-LANCASTER-CASS-SALINE-
INCLUDING THE CITIES OF...ONAWA...BLOOMFIELD...HARTINGTON...
PENDER...NELIGH...PIERCE...WAYNE...ALBION...NORFOLK...STANTON...
WEST POINT...BLAIR...OMAHA...BELLEVUE...LINCOLN...PLATTSMOUTH...
CRETE
1018 PM CST SUN NOV 26 2006

...FREEZING RAIN ADVISORY REMAINS IN EFFECT UNTIL 8 AM CST
MONDAY...

A FREEZING RAIN ADVISORY REMAINS IN EFFECT UNTIL 8 AM CST MONDAY.

COLD ARCTIC AIR IS OVER THE AREA. RAIN FREEZING ON IMPACT TO
SURFACES IS EXPECTED TO OCCUR THROUGH THE EARLY MORNING HOURS.
AMOUNTS ARE EXPECTED TO BE UNDER A COUPLE OF OF HUNDREDTHS OF AN
INCH...BUT SIDEWALKS AND UNTREATED ROADWAYS MAY BECOME SLICK
THROUGH THE OVERNIGHT HOURS.

A FREEZING RAIN ADVISORY MEANS THAT PERIODS OF FREEZING RAIN OR
FREEZING DRIZZLE WILL CAUSE TRAVEL DIFFICULTIES. BE PREPARED FOR
SLIPPERY ROADS. SLOW DOWN AND USE CAUTION WHILE DRIVING.

Here's the latest satellite image: http://weather.unisys.com/satellite/sat ... mw_inv.gif

With the front exiting the area already, the likelihood for precipitation is lowered, although it wouldn't be too surprising to see some drizzle.

However, with that said, the current observations show IFR to LIFR conditions throughout eastern Nebraska and western Iowa. It should be pointed out, however, that some freezing rain is falling in southeastern South Dakota where wrap-around moisture is producing the precipitation.

Now, the question: why freezing rain/drizzle rather than just normal rain/drizzle?

Well, check out the Omaha sounding:


For those who have never looked at a sounding before, this is a temperature, dewpoint, and wind profile of the atmosphere, as recording from an instrument-equipped balloon launched from Omaha this evening.

The blue trace is the dewpoint whereas the red trace is the temperature.

The bottom of the traces is the conditions at the surface--near freezing. As the night progresses, this temperature will continue to drop. Now, notice that the temperatures from about 2000-1000ft are actually above freezing?

Also, where the red and blue traces are close to each other shows where the relative humidity is high--a good indication where clouds are. This sounding shows clouds from about 7000ft to near the ground. (Hence the reports of fog)

Now with that in mind, if the clouds produce precipitation, it will begin as liquid, since the temperatures between 2000ft and the cloud tops are above freezing. The water droplet will encounter freezing conditions for the last 2000ft before contacting the ground. With the average speed of a raindrop in this situation at around 10 mph (give or take a few), or about 880ft/minute, that gives our raindrop about 2 1/2 minutes to freeze before hitting the ground--not really enough time to cool the entire drop below the freezing point. (Think about how quick it take to freeze an icecube in your freezer.)

So, the moment our raindrop hits the ground, it's almost at it's freezing point, but the surface of the ground is at the freezing point. The contact is enough to quickly finish the freezing process. Bottom line--the raindrop falls as a liquid, hits as a liquid, and quickly changes over to ice, creating clear icing.


Now, clear icing is not only a problem for us ground folk. It also hamper aircraft operations. (Therefore, those last-minute flights home from grandma's house may be delayed.)

The Aviation Weather Center (part of NOAA) has an airmet out for occasional moderate rime icing. Rime icing is a white or milky opaque granular deposit of ice. It occurs when supercooled water droplets strike an object at temperatures at or below freezing. Factors favoring the formation of rime are small drop size, slow accretion, a high degree of supercooling, and rapid dissipation of latent heat of fusion. Rime is a result of freezing drizzle and looks like frost in a freezer. Rime icing which forms on aircraft can seriously distort airfoil shape, therefore diminishing lift and performance. Rime icing is more likely to form in stratus-type clouds with temperatures between 0°C and minus 22°C. When formed in cumuliform-type clouds, temperatures range from minus 9°C to minus 15°C and are accompanied by clear icing which is then termed mixed icing.

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WAUS43 KKCI 270245
CHIZ WA 270245
AIRMET ZULU FOR ICE AND FRZLVL VALID UNTIL 270900
AIRMET ICE...NE KS
FROM 50SSW FSD TO 30NNW OVR TO 20SSE HLC TO 30E LAA TO GLD TO BFF
TO 50SSW FSD
OCNL MOD RIME/MXD ICGICIP BTN SFC AND 060. CONDS CONTG BYD 09Z
ENDG NR 15Z.
FRZLVL...AT SFC DAKOTAS/NRN 2/3RDS NEB/NW HLF MN/NRN LS
 SLPG TO 125 SE KY
 AT SFC NR AND N LN 55ESE SNY-OBH-RWF-30W DLH-YQT
 MULT FRZLVLS 030-080 SRN HLF SD/NEB XCP SE NEB/NW KS
 040 NR LN 70SW RAP-70NW RWF-DLH-30N SSM
 080 NR LN BFF-40SSE ANW-40S GRB-55S YVV
 120 NR LN 30N RZC-40N PXV-CVG-HNN

For what it's worth, AWC also has an airmet out for IFR (instrument flight rules) for this area. That means that the visibility and/or cloud ceiling is low enough that pilots will resort to flying by instruments rather than sight.

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WAUS43 KKCI 270245
CHIS WA 270245
AIRMET SIERRA FOR IFR VALID UNTIL 270900
AIRMET IFR...SD NE KS MN IA MO WI LM LS MI LH IL
FROM 30SW YQT TO SSM TO YVV TO DXO TO DBQ TO 60ESE OVR TO ICT TO
50W LBL TO GLD TO BFF TO 50NW FSD TO 30SW YQT
OCNL CIG BLW 010/VIS BLW 3SM CLDS/PCPN/BR/FG. CONDS CONTG BYD 09Z
THRU 15Z.

Bottom line, with the low clouds, low visibility, and possible freezing precip, travel by air or by ground may be hampered in the area tonight and tomorrow morning.

[senorpepr]

Sure enough... the FZRA advisory has been dropped for much of Nebraska, although South Dakota/Iowa can still see some action.