Radar Rules of Thumb

Annual Hampton Roads Radar Conference (USN, USAF, NWS)

NOTE: This guide summarizes the input of experienced W-88 operators at the Radar Conference. Some rules are local and intended just for the Hampton Roads area, such as the "Norfolk split". Others can be applied generally. Dr. Walt McKeown, Naval Atlantic Meteorology and Oceanography Center

From USAF SSGT Daniel Tucker (Ft. Eustis, VA)

Using the 0.5 Reflectivity and Velocity maps lets you track and time the passage of gust fronts.
In winter, cells coming from the west normally collapse when they move over the colder James River.
In summer, cells coming from the west normally build when they move over the warmer James River.
In determining snow versus rain and the resulting accumulations, Reflectivity can be unrepresentative.
In a warm over-running situation (TC), VIL and STP products provide the best indication for deciphering the embedded thunderstorms.

From USN AG1 Kotyk (NLMOC, Norfolk, VA)

In the summer, the cooler waters of Chesapeake Bay will affect the trajectory of thunderstorms causing the infamous "Norfolk-split". A line of storms will divide - one section going northeastward over the Peninsula, the other going southeastward toward Elizabeth City.
The VAD Wind Profile is good for determining the presence of inversions.
The combined use of LPATS (Lightning Position and Tracking System) with NEXRAD helps to discern embedded thunderstorms that NEXRAD may miss. This can also help to determine the difference between a heavy downpour and a thunderstorm.

From NWS Science Officer Carven Scott (Anchorage, AL.)

In general, atmospheric processes causing high spectrum width values also generate turbulence. 12 - 13 kts ----- moderate turbulence 15 - 16 kts ----- severe turbulence

From NWS Forecaster Neil Stuart (Wakefield, VA)

Straight-line wind fields

Check area soundings and surface obs. for a marine layer or inversion that would prevent strong winds from reaching the surface
More often than not, a 54 dBZ or above higher than 33kft core hitting ground will cause localized wind damage, possible severe hail depending on freezing level and wet bulb zero level
Stand ready to investigate further and warn quickly if there are indications of bowing or a rear inflow notch or if a linear feature is tracking >40 mph.
Where you see lines of storms moving perpendicular to radials, use SRM to investigate straight-line winds.
VAD wind profile (VWP) will show descending jets and shear characteristics of the environment.

Hail

Base reflectivity >55 dBZ can be representative of hail
Base reflectivitiy <60 dBZ usually mean more rain than hail, but hail cores aloft can still produce severe hail, when base reflectivities are <60 dBZ.
Base reflectivities >60 dBZ often have some hail associated with them, and >64 dBZ will often result in hail near 3/4" diameter, depending on the freezing level.
In the middle of the summer, >65 dBZ must extend above the freezing or wet bulb zero level. Otherwise hail will usually melt before reaching ground (depending on the freezing and wet bulb zero level).
Layer Reflectivity/Max will highlight the hail potential if >54 dBZ extends above 24kft or 33kft (depending on the freezing and wet bulb zero level).
Hail algorithm has its strengths and weaknesses. Don't take the forecast hail sizes literally. Radar will highlight the storms with largest hail.
VIL thresholds depend on the atmospheric characteristics. Once a hail report is given, it is useful to match the VIL value to the reported hail size.
Composite reflectivity shows the maximum reflectivity in the storm, and can better represent hail potential if a hail core is aloft.

Tornadoes

Although the tornado detection algorithm sometimes gives false alarms, it is robust enough that close investigation of the storm is necessary if a TVS or ETVS appears.
Mesocyclone detection algorithm can give false alarms, missing shallow mesocyclones beyond 35 miles.
Symmetric circulations vertically correlated through at least 2 consecutive elevation angles are most dangerous, even though tropical convection can be shallow and circulations may be only at the lowest elevation angle
Look for mesoscale low pressure centers in surface obs. plots for heightened chances of tornadic activity

Heavy rain and flooding

Rainfall estimates are reasonably accurate, especially in maxima produced by convection.
Stratiform rainfall estimates are often too little, sometimes half or less of observed.
Investigate area soundings and other data for precipitable water and other parameters to evaluate the potential for heavy rain.

Miscellaneous

The radar depicts localized boundaries very well, including thunderstorm outflow boundaries, James River and Bay-induced boundaries, windshifts, cold front and sea breeze boundaries. Sometimes on humid breezy days, cloud streets can be seen
Colliding boundaries and intersecting boundaries are dangerous

Compiled by:

Dr. Walt McKeown
Naval Atlantic Meteorology and Oceanography Center
Norfolk, VA
757-445-2546
walt.mckeown@nlmoc.navy.mil

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