Pre-Frontal Shear Line?

[jgf]

Copied below is today's WPC Tropical Discussion (which is not the same as the Tropical Weather Discussion issued by the NHC..).., and as you can see it is dominated by discussion of a "pre-frontal shear line". I know what "shear" is - i studied mechanics in graduate school - and I have this definition from the AMS:

"Shear Line: In meteorology, a line or narrow zone across which there is an abrupt change in the horizontal wind component parallel to this line; a line of maximum horizontal wind shear."

and also this from Wikepedia:

"Stationary fronts will either dissipate after several days or devolve into shear lines, but can change into a cold or warm front if conditions aloft change causing a driving of one air mass or the other. Stationary fronts are marked on weather maps with alternating red half-circles and blue spikes pointing in opposite directions, indicating no significant movement.

When stationary fronts become smaller in scale, degenerating to a narrow zone where wind direction changes over a short distance, they become known as shear lines.[29] If the shear line becomes active with thunderstorms, it may support formation of a tropical storm or a regeneration of the feature back into a stationary front. A shear line is depicted as a line of red dots and dashes.[13]"

This shear line is not depicted on any surface analysis or surface forecast that I have seen. They seem to have a pretty good handle on where it is, and where it will be in the future...

i have attached a surface analysis below the discussion.., 

So, I have a few questions:

what data or observations, or model results,  are they using to determine the existence of a shear line? 

would it be expressed in surface winds.., or winds higher up.., or both?

How can i see it myself?

FXCA20 KWBC 261149
PMDCA 

TROPICAL DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
648 AM EST FRI JAN 26 2018

PRELIMINARY DISCUSSION FOR PUERTO RICO AND THE USVI FROM JAN 26/06
UTC: BUILDING POLAR RIDGE OVER THE WESTERN ATLANTIC/EASTERN USA IS
DRIVING A POLAR FRONT ACROSS BERMUDA-THE BAHAMAS EARLY THIS
MORNING. THIS TRIGGERED A PREFRONTAL SHEAR LINE THAT IS SURGING
ACROSS THE DOMINICAN REPUBLIC. STRONG BOUNDARY LAYER WINDS
ACCOMPANY THIS FEATURE...WITH SHIP REPORTS NEAR THE TURKS AND
CAICOS SHOWING WINDS OF 25-30KT. THIS IS ENHANCING LOW LEVEL
MOISTURE CONVERGENCE ALONG THE SHEAR LINE...WITH SATELLITE DERIVED
PWAT ANALYSIS SHOWING MOISTURE POOLING ACROSS NORTHERN HISPANIOLA.
HOWEVER...PERSISTENT MID LEVEL RIDGE AND ASSOCIATED TRADE WINDS
CAP CONTINUES TO INHIBIT DEEP CONVECTION IN ASSOCIATION WITH THIS
FEATURE. AHEAD OF THE SHEAR LINE...A FRESH NORTHEASTERLY FLOW AND
A PWAT MINIMA ENVELOP THE FORECAST AREA.

THE SURFACE FRONT IS TO SAG JUST NORTH OF THE FORECAST AREA EARLY
ON SATURDAY MORNING...WHILE TRAILING ACROSS THE BAHAMAS TO THE
FLORIDA PENINSULA. BOUNDARY IS TO REMAIN NEARLY STATIONARY THROUGH
EARLY NEXT WEEK...WHEN IT IS EXPECTED TO FRONTOLIZE. THE
PREFRONTAL SHEAR LINE MOVES ACROSS THE MONA PASSAGE TO NORTHWEST
PUERTO RICO THIS MORNING...PULLING ACROSS SAN JUAN BY 18-21 UTC
TODAY. THIS WILL BE ACCOMPANIED BY A 20-30KT WIND SURGE...COLD
ADVECTION AND A LOW LEVEL CONVERGENT PATTERN. THE SHEAR LINE
ENTERING THE SAN JUAN METRO AREA WILL COINCIDE WITH PERIOD OF MAX
HEATING...ENHANCING CONVECTIVE INSTABILITY AS THIS FEATURE CROSSES
THE ISLAND. THE SHEAR LINE IS THEN FORECAST TO MEANDER BETWEEN THE
VIRGIN ISLES-EASTERN PUERTO RICO THROUGH SATURDAY MORNING...MOVING
FARTHER EAST ACROSS THE ISLAND CHAIN LATER IN THE DAY. AS IT PULLS
AWAY AND RIDGE STRENGTHENS OVER THE ATLANTIC THE LOW/MID LEVEL
WINDS ARE TO GRADUALLY VEER TO THE EAST DURING THE WEEKEND. STRONG
BOUNDARY LAYER WINDS ARE TO PERSIST THROUGH MONDAY...TO CONTINUE
FAVORING A COOL ADVECTIVE PATTERN AND GENERATION OF TRADE WIND
SHOWERS ACROSS THE VIRGIN ISLES INTO PUERTO RICO.

UNDER THE INFLUENCE OF THE WIND SURGE/SHEAR LINE CONVERGENCE AND
ASSOCIATED COLD ADVECTIVE PATTERN...EXPECTING A MORE SEASONAL
PATTERN OF TRADE WIND SHOWERS TO AFFECT PUERTO RICO AND THE VIRGIN
ISLES DURING THE NEXT FEW DAYS. FURTHERMORE... THE STRONGER
EASTERLY TRADES ARE TO ALSO FAVOR GENERATION OF STREAMERS THAT ARE
TO CONVERGE ACROSS VIEQUES...CULEBRA AND EASTERN PORTIONS OF
PUERTO RICO. THESE WILL COMBINE WITH PERSISTENT COOL ADVECTIVE
PATTERN AND MEANDERING SHEAR LINE TO LIKELY FAVOR MODERATE TO
HEAVY RAINFALL AMOUNTS ACROSS THE VIRGIN ISLES AND EASTERN PUERTO
RICO/SAN JUAN METRO AREA THIS AFTERNOON THROUGH SATURDAY MORNING.
LIGHT TO MODERATE RAINFALL AMOUNTS ARE THEN EXPECTED THROUGHOUT
THE WEEKEND/EARLY NEXT WEEK...WITH MOST ACTIVE CLUSTERING ACROSS
THE EASTERN THIRD OF PUERTO RICO...WITH LOCALIZED MAXIMA OVER
SIERRA DE LUQUILLO/EL YUNQUE.


DAVISON/GALVEZ...WPC (USA) 

$$