WRAL WeatherCenter Blog

Today (Feb 1) looking at the doppler there was yellow and red showing up around Sampson county. Was that heavy rain or a mix of rain and sleet. I do know when rain storms are around it is heavy rain was that the case today? And how many inches of rain fell?

Posted February 8, 2007

MIKE MOSS SAYS:    Judy,    There were two things going on that helped make radar reflectivities higher across the region you're asking about. First is that there was a band of higher precipitation rates across that region compared to areas farther north. In addition, however, the radar colors were even more intense (the yellows and reds you cited) along a line where snow was in the process of changing to rain due to warmer air spreading into the region from the south. Under the right conditions, this can create a situation called "bright banding" in which the radar is cutting through a layer in which snowflakes are just beginning to melt, so that the flakes and crystals are still rather large but are becoming coated with a bit of liquid water. Once the flakes melt completely, the resulting droplets are falling faster and are considerably smaller.

"Dry" snowflakes that are well below freezing are rather poor reflectors of radar energy, so a radar beam passing through an area that is all dry snow will usually show rather weak echoes. The fully melted raindrops at lower elevations in the region where melting was going on would also produce fairly weak echoes due to their faster speed and greater spacing, and the fact that the drops were fairly small. The layer in between, however, can produce noticeably stronger returns because of the liquid water coating the still rather large and fairly slow snow flakes and crystals.

Likewise, even though precipitation rates were a little higher in some places south and east of the bright band you noticed, remember that the radar beam is climbing higher and higher at increasing distances from the radar, so at a larger distance it may be at an elevation high enough to be back into mainly "dry" snow, and thus the reflectivies appear weaker again.

For this event, here are some liquid/snow measurements from a few stations around the area (Feb 1st only - doesn't include additional rain that fell early Feb 2nd). Note that the liquid total includes any moisture that fell as snow or sleet):

RDU Airport - .44"/.6"
Fayetteville Airport - 1.02" (official snowfall amount not reported, but about 1")
Sanford Airport - .71" (around 1" snow)
Pope AFB - .79" (around 1" snow)
Louisburg Airport - .39" (around .5" inch snow)
Erwin Airport - .93" (around 1" snow)
Rocky Mt-Wilson Airport - .53" (around 1" snow)

You can read a much more detailed overview of this event in a summary posted by the Raleigh National Weather Service Office at

http://www4.ncsu.edu/~nwsfo/storage/cases/20070201/




Comments

Please with your WRAL.com account to comment on this story. You also will need a Facebook account to comment.

Oldest First
View all